diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 70aa6b7..cd555d8 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -46,16 +46,21 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-latest, windows-latest]
+        include:
+          - os: ubuntu-latest
+            data_sep: ":"
+          - os: windows-latest
+            data_sep: ";"
     steps:
       - uses: actions/checkout@v4
       - uses: actions/setup-python@v5
         with:
           python-version: "3.11"
       - run: pip install -e ".[dev]"
+      - run: pybabel compile -d src/nexus_sync/locale -D nexus
       - run: pyinstaller --onefile src/nexus_sync/client/__main__.py --name nexus-sync-client
       - run: pyinstaller --onefile src/nexus_sync/server/__main__.py --name nexus-sync-server
-      - run: pyinstaller --onefile src/nexus_sync/cli/__main__.py --name nexus-cli
+      - run: pyinstaller --onefile src/nexus_sync/cli/__main__.py --name nexus-cli --add-data "src/nexus_sync/locale${{ matrix.data_sep }}nexus_sync/locale"
       - uses: actions/upload-artifact@v4
         with:
           name: nexus-sync-${{ matrix.os }}
diff --git a/Makefile b/Makefile
index 0948247..eabae8f 100644
--- a/Makefile
+++ b/Makefile
@@ -1,4 +1,11 @@
-.PHONY: all client server cli test clean
+.PHONY: all client server cli test clean i18n-extract i18n-init i18n-update i18n-compile
+
+# CLI localization (nexus-cli strings only; daemon/server logs stay in English).
+LOCALE_DIR := src/nexus_sync/locale
+POT := $(LOCALE_DIR)/nexus.pot
+LANG ?= ru
+# PyInstaller --add-data separator is ':' on Unix, ';' on Windows.
+LOCALE_DATA := $(LOCALE_DIR):nexus_sync/locale
 
 all: client server cli
 
@@ -8,8 +15,25 @@ client:
 server:
 	pyinstaller --onefile src/nexus_sync/server/__main__.py --name nexus-sync-server
 
-cli:
-	pyinstaller --onefile src/nexus_sync/cli/__main__.py --name nexus-cli
+cli: i18n-compile
+	pyinstaller --onefile src/nexus_sync/cli/__main__.py --name nexus-cli \
+		--add-data "$(LOCALE_DATA)"
+
+# Rebuild the message template from strings wrapped in _()/gettext()/ngettext().
+i18n-extract:
+	pybabel extract -F babel.cfg -k _ -o $(POT) src
+
+# Create a catalog for a new language, e.g. `make i18n-init LANG=de`.
+i18n-init: i18n-extract
+	pybabel init -i $(POT) -d $(LOCALE_DIR) -D nexus -l $(LANG)
+
+# Merge new/changed strings into existing catalogs.
+i18n-update: i18n-extract
+	pybabel update -i $(POT) -d $(LOCALE_DIR) -D nexus
+
+# Compile .po catalogs to the .mo files bundled with nexus-cli.
+i18n-compile:
+	pybabel compile -d $(LOCALE_DIR) -D nexus
 
 test:
 	pytest
diff --git a/babel.cfg b/babel.cfg
new file mode 100644
index 0000000..090e5f4
--- /dev/null
+++ b/babel.cfg
@@ -0,0 +1,5 @@
+# Babel extraction config for CLI localization.
+# Paths are relative to the extraction root passed to pybabel (the `src` dir).
+# Only strings wrapped in _()/gettext()/ngettext() are extracted, so logger.*
+# messages are never picked up.
+[python: **.py]
diff --git a/docs/cli.md b/docs/cli.md
index 115c0e4..b3fd5c2 100644
--- a/docs/cli.md
+++ b/docs/cli.md
@@ -61,10 +61,37 @@ nexus-cli --json client linux-client
 nexus-cli --json command cmd_123
 ```
 
+## Localization
+
+CLI output is localized with `gettext` (catalogs managed by Babel). Only
+user-facing CLI strings are translatable; daemon/server log messages are
+deliberately left untranslated.
+
+Select a language with the `NEXUS_SYNC_LANG` env var (falls back to the system
+locale, then to the source English strings):
+
+```bash
+NEXUS_SYNC_LANG=ru nexus-cli command cmd_123
+```
+
+Translation sources live in `src/nexus_sync/locale/<lang>/LC_MESSAGES/nexus.po`.
+Workflow (requires `pip install -e ".[dev]"`):
+
+```bash
+make i18n-extract           # rebuild the .pot template from _()-wrapped strings
+make i18n-update            # merge new/changed strings into existing catalogs
+make i18n-init LANG=de      # start a new language
+# edit the .po file, then:
+make i18n-compile           # build the .mo files shipped with the binary
+```
+
+`make cli` compiles catalogs automatically and bundles them into the binary.
+
 ## Build standalone binary
 
 ```bash
 make cli
 ```
 
-This creates `dist/nexus-cli` through PyInstaller.
+This creates `dist/nexus-cli` through PyInstaller (with localization catalogs
+bundled via `--add-data`).
diff --git a/pyproject.toml b/pyproject.toml
index d512409..1527625 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -25,7 +25,8 @@ dev = [
     "pyinstaller>=6.0",
     "mypy>=1.0",
     "types-PyYAML>=6.0.12",
-    "pre-commit>=4.0"
+    "pre-commit>=4.0",
+    "Babel>=2.14.0"
 ]
 
 [project.scripts]
@@ -34,6 +35,9 @@ nexus-cli = "nexus_sync.cli.__main__:main"
 [tool.setuptools.packages.find]
 where = ["src"]
 
+[tool.setuptools.package-data]
+nexus_sync = ["locale/**/*.mo"]
+
 [tool.black]
 line-length = 100
 target-version = ["py311"]
diff --git a/src/nexus_sync/cli/__main__.py b/src/nexus_sync/cli/__main__.py
index 0e1010e..76e2c0c 100644
--- a/src/nexus_sync/cli/__main__.py
+++ b/src/nexus_sync/cli/__main__.py
@@ -8,6 +8,9 @@ from collections.abc import Callable
 from types import TracebackType
 from typing import Any, Protocol, Self
 
+from nexus_sync import i18n
+from nexus_sync.i18n import _
+
 DEFAULT_SERVER_URL = "http://127.0.0.1:5852"
 API_PREFIX = "/api/v1"
 JsonObject = dict[str, Any]
@@ -53,62 +56,62 @@ def request_json(
             body = response.read()
     except urllib.error.HTTPError as error:
         detail = error.read().decode(errors="replace")
-        raise CLIError(f"HTTP {error.code}: {detail}") from error
+        raise CLIError(_("HTTP {code}: {detail}").format(code=error.code, detail=detail)) from error
     except urllib.error.URLError as error:
-        raise CLIError(f"request failed: {error.reason}") from error
+        raise CLIError(_("request failed: {error}").format(error=error.reason)) from error
     except OSError as error:
-        raise CLIError(f"request failed: {error}") from error
+        raise CLIError(_("request failed: {error}").format(error=error)) from error
 
     try:
         result = json.loads(body.decode())
     except json.JSONDecodeError as error:
-        raise CLIError(f"server returned invalid JSON: {error}") from error
+        raise CLIError(_("server returned invalid JSON: {error}").format(error=error)) from error
     if not isinstance(result, dict):
-        raise CLIError("server returned JSON that is not an object")
+        raise CLIError(_("server returned JSON that is not an object"))
     return result
 
 
 def format_clients(payload: JsonObject) -> str:
     clients = payload.get("clients", [])
     if not isinstance(clients, list) or not clients:
-        return "clients:\n- none"
+        return _("clients:") + "\n- " + _("none")
 
-    lines = ["clients:"]
+    lines = [_("clients:")]
     for item in clients:
         if not isinstance(item, dict):
             continue
         parts = [
-            str(item.get("id", "<unknown>")),
-            str(item.get("platform", "unknown")),
-            str(item.get("hostname", "unknown")),
-            f"version={item.get('version', 'unknown')}",
-            f"last_seen={item.get('last_seen_at', 'unknown')}",
+            str(item.get("id", _("<unknown>"))),
+            str(item.get("platform", _("unknown"))),
+            str(item.get("hostname", _("unknown"))),
+            f"version={item.get('version', _('unknown'))}",
+            f"last_seen={item.get('last_seen_at', _('unknown'))}",
         ]
         lines.append(f"- {'  '.join(parts)}")
         commands = _command_names(item.get("available_commands", []))
         if commands:
-            lines.append(f"  commands: {', '.join(commands)}")
+            lines.append(f"  {_('commands:')} {', '.join(commands)}")
     return "\n".join(lines)
 
 
 def format_client(payload: JsonObject) -> str:
     lines = [
-        f"id: {payload.get('id', '<unknown>')}",
-        f"hostname: {payload.get('hostname', 'unknown')}",
-        f"platform: {payload.get('platform', 'unknown')}",
-        f"version: {payload.get('version', 'unknown')}",
-        f"created_at: {payload.get('created_at', 'unknown')}",
-        f"last_seen_at: {payload.get('last_seen_at', 'unknown')}",
-        "available_commands:",
+        _field_line("id", payload.get("id", _("<unknown>"))),
+        _field_line("hostname", payload.get("hostname", _("unknown"))),
+        _field_line("platform", payload.get("platform", _("unknown"))),
+        _field_line("version", payload.get("version", _("unknown"))),
+        _field_line("created_at", payload.get("created_at", _("unknown"))),
+        _field_line("last_seen_at", payload.get("last_seen_at", _("unknown"))),
+        _("available_commands:"),
     ]
     commands = payload.get("available_commands", [])
     if not isinstance(commands, list) or not commands:
-        lines.append("- none")
+        lines.append("- " + _("none"))
         return "\n".join(lines)
 
     for command in commands:
         if isinstance(command, dict):
-            name = command.get("name", "<unknown>")
+            name = command.get("name", _("<unknown>"))
             description = command.get("description", "")
             suffix = f" - {description}" if description else ""
             lines.append(f"- {name}{suffix}")
@@ -116,7 +119,7 @@ def format_client(payload: JsonObject) -> str:
 
 
 def format_command(payload: JsonObject, *, queued: bool = False) -> str:
-    lines = ["queued command:" if queued else "command:"]
+    lines = [_("queued command:") if queued else _("command:")]
     for key in (
         "id",
         "client_id",
@@ -129,43 +132,45 @@ def format_command(payload: JsonObject, *, queued: bool = False) -> str:
         "finished_at",
     ):
         if key in payload:
-            lines.append(f"{key}: {payload.get(key)}")
+            lines.append(_field_line(key, payload.get(key)))
 
     result = payload.get("result")
     if isinstance(result, dict):
-        lines.append("result:")
+        lines.append(_("result:"))
         if "status" in result:
-            lines.append(f"  status: {result.get('status')}")
+            lines.append("  " + _field_line("status", result.get("status")))
         if "return_code" in result:
-            lines.append(f"  return_code: {result.get('return_code')}")
-        lines.append("  stdout:")
+            lines.append("  " + _field_line("return_code", result.get("return_code")))
+        lines.append("  " + _("stdout:"))
         lines.extend(_indent_block(str(result.get("stdout", ""))))
-        lines.append("  stderr:")
+        lines.append("  " + _("stderr:"))
         lines.extend(_indent_block(str(result.get("stderr", ""))))
     return "\n".join(lines)
 
 
 def build_parser() -> argparse.ArgumentParser:
-    parser = argparse.ArgumentParser(prog="nexus-cli", description="CLI for nexus-sync server API")
-    parser.add_argument(
-        "--server-url", "-s", default=DEFAULT_SERVER_URL, help="nexus-sync server URL"
+    parser = argparse.ArgumentParser(
+        prog="nexus-cli", description=_("CLI for nexus-sync server API")
     )
-    parser.add_argument("--list", action="store_true", help="list clients")
-    parser.add_argument("--json", action="store_true", help="print raw JSON response")
+    parser.add_argument(
+        "--server-url", "-s", default=DEFAULT_SERVER_URL, help=_("nexus-sync server URL")
+    )
+    parser.add_argument("--list", action="store_true", help=_("list clients"))
+    parser.add_argument("--json", action="store_true", help=_("print raw JSON response"))
 
     subparsers = parser.add_subparsers(dest="resource")
-    client = subparsers.add_parser("client", help="show client info or queue a command")
-    client.add_argument("id", help="client id")
-    client.add_argument("--run-command", metavar="NAME", help="queue a command for this client")
+    client = subparsers.add_parser("client", help=_("show client info or queue a command"))
+    client.add_argument("id", help=_("client id"))
+    client.add_argument("--run-command", metavar="NAME", help=_("queue a command for this client"))
     client.add_argument(
         "--timeout-seconds",
         type=int,
         default=30,
-        help="command timeout in seconds for --run-command",
+        help=_("command timeout in seconds for --run-command"),
     )
 
-    command = subparsers.add_parser("command", help="show command execution info")
-    command.add_argument("id", help="command id")
+    command = subparsers.add_parser("command", help=_("show command execution info"))
+    command.add_argument("id", help=_("command id"))
     return parser
 
 
@@ -174,6 +179,7 @@ def main(
     *,
     requester: Requester = request_json,
 ) -> int:
+    i18n.setup()
     parser = build_parser()
     args = parser.parse_args(argv)
     server_url = str(args.server_url).rstrip("/")
@@ -215,7 +221,7 @@ def main(
             _print_payload(payload, raw_json=args.json, formatter=format_command)
             return 0
     except CLIError as error:
-        print(f"nexus-cli error: {error}", file=sys.stderr)
+        print(_("nexus-cli error: {error}").format(error=error), file=sys.stderr)
         return 1
 
     parser.print_help()
@@ -234,6 +240,30 @@ def _print_payload(
     print(formatter(payload))
 
 
+def _field_line(key: str, value: object) -> str:
+    return f"{_field_label(key)}: {value}"
+
+
+def _field_label(key: str) -> str:
+    labels = {
+        "id": _("id"),
+        "client_id": _("client_id"),
+        "hostname": _("hostname"),
+        "platform": _("platform"),
+        "version": _("version"),
+        "created_at": _("created_at"),
+        "last_seen_at": _("last_seen_at"),
+        "kind": _("kind"),
+        "name": _("name"),
+        "status": _("status"),
+        "timeout_seconds": _("timeout_seconds"),
+        "delivered_at": _("delivered_at"),
+        "finished_at": _("finished_at"),
+        "return_code": _("return_code"),
+    }
+    return labels.get(key, key)
+
+
 def _command_names(commands: object) -> list[str]:
     if not isinstance(commands, list):
         return []
@@ -246,7 +276,7 @@ def _command_names(commands: object) -> list[str]:
 
 def _indent_block(value: str) -> list[str]:
     if not value:
-        return ["    <empty>"]
+        return ["    " + _("<empty>")]
     return [f"    {line}" if line else "" for line in value.rstrip("\n").splitlines()]
 
 
diff --git a/src/nexus_sync/i18n.py b/src/nexus_sync/i18n.py
new file mode 100644
index 0000000..949120a
--- /dev/null
+++ b/src/nexus_sync/i18n.py
@@ -0,0 +1,56 @@
+"""CLI localization (i18n).
+
+User-facing CLI strings are wrapped in :func:`_` so they can be translated.
+Log messages (``logger.*``) are intentionally left unwrapped and always stay
+in English for grep-ability and operations.
+
+Translation only becomes active after :func:`setup` is called (done in the CLI
+entry points). Until then, and whenever no catalog matches the requested
+language, ``gettext`` falls back to returning the original (English) message.
+"""
+
+from __future__ import annotations
+
+import gettext as _gettext
+import os
+import sys
+from pathlib import Path
+
+DOMAIN = "nexus"
+LANG_ENV = "NEXUS_SYNC_LANG"
+
+_translation: _gettext.NullTranslations = _gettext.NullTranslations()
+
+
+def _locale_dir() -> str:
+    """Locate the compiled message catalogs, both in-source and inside a PyInstaller bundle."""
+    bundled = getattr(sys, "_MEIPASS", None)
+    if bundled is not None:
+        return os.path.join(bundled, "nexus_sync", "locale")
+    return str(Path(__file__).resolve().parent / "locale")
+
+
+def setup(lang: str | None = None) -> None:
+    """Activate the message catalog for ``lang``.
+
+    When ``lang`` is ``None`` the ``NEXUS_SYNC_LANG`` env var is consulted, and
+    failing that the system locale (``LANGUAGE``/``LANG``/...) is used. Missing
+    catalogs fall back silently to the original English strings.
+    """
+    global _translation
+    if lang is None:
+        lang = os.environ.get(LANG_ENV) or None
+    languages = [lang] if lang else None
+    _translation = _gettext.translation(DOMAIN, _locale_dir(), languages=languages, fallback=True)
+
+
+def gettext(message: str) -> str:
+    return _translation.gettext(message)
+
+
+def ngettext(singular: str, plural: str, n: int) -> str:
+    return _translation.ngettext(singular, plural, n)
+
+
+# Conventional alias used to mark translatable strings; recognized by pybabel.
+_ = gettext
diff --git a/src/nexus_sync/locale/ru/LC_MESSAGES/nexus.po b/src/nexus_sync/locale/ru/LC_MESSAGES/nexus.po
new file mode 100644
index 0000000..b4e9b16
--- /dev/null
+++ b/src/nexus_sync/locale/ru/LC_MESSAGES/nexus.po
@@ -0,0 +1,192 @@
+# Russian translations for nexus-sync.
+# Copyright (C) 2026 ORGANIZATION
+# This file is distributed under the same license as the nexus-sync project.
+#
+msgid ""
+msgstr ""
+"Project-Id-Version: nexus-sync 0.1.0\n"
+"Report-Msgid-Bugs-To: EMAIL@ADDRESS\n"
+"POT-Creation-Date: 2026-06-24 22:58+0300\n"
+"PO-Revision-Date: 2026-06-24 23:01+0300\n"
+"Last-Translator: nexus-sync\n"
+"Language: ru\n"
+"Language-Team: ru <LL@li.org>\n"
+"Plural-Forms: nplurals=3; plural=(n%10==1 && n%100!=11 ? 0 : n%10>=2 && "
+"n%10<=4 && (n%100<10 || n%100>=20) ? 1 : 2);\n"
+"MIME-Version: 1.0\n"
+"Content-Type: text/plain; charset=utf-8\n"
+"Content-Transfer-Encoding: 8bit\n"
+"Generated-By: Babel 2.18.0\n"
+
+#: src/nexus_sync/cli/__main__.py:59
+#, python-brace-format
+msgid "HTTP {code}: {detail}"
+msgstr ""
+
+#: src/nexus_sync/cli/__main__.py:61 src/nexus_sync/cli/__main__.py:63
+#, python-brace-format
+msgid "request failed: {error}"
+msgstr "запрос не выполнен: {error}"
+
+#: src/nexus_sync/cli/__main__.py:69
+#, python-brace-format
+msgid "server returned invalid JSON: {error}"
+msgstr "сервер вернул некорректный JSON: {error}"
+
+#: src/nexus_sync/cli/__main__.py:72
+msgid "server returned JSON that is not an object"
+msgstr "сервер вернул JSON, который не является объектом"
+
+#: src/nexus_sync/cli/__main__.py:79 src/nexus_sync/cli/__main__.py:81
+msgid "clients:"
+msgstr "клиенты:"
+
+#: src/nexus_sync/cli/__main__.py:79 src/nexus_sync/cli/__main__.py:111
+msgid "none"
+msgstr "нет"
+
+#: src/nexus_sync/cli/__main__.py:86 src/nexus_sync/cli/__main__.py:101
+#: src/nexus_sync/cli/__main__.py:116
+msgid "<unknown>"
+msgstr "<неизвестно>"
+
+#: src/nexus_sync/cli/__main__.py:87 src/nexus_sync/cli/__main__.py:88
+#: src/nexus_sync/cli/__main__.py:89 src/nexus_sync/cli/__main__.py:90
+#: src/nexus_sync/cli/__main__.py:102 src/nexus_sync/cli/__main__.py:103
+#: src/nexus_sync/cli/__main__.py:104 src/nexus_sync/cli/__main__.py:105
+#: src/nexus_sync/cli/__main__.py:106
+msgid "unknown"
+msgstr "неизвестно"
+
+#: src/nexus_sync/cli/__main__.py:95
+msgid "commands:"
+msgstr "команды:"
+
+#: src/nexus_sync/cli/__main__.py:107
+msgid "available_commands:"
+msgstr "доступные команды:"
+
+#: src/nexus_sync/cli/__main__.py:124
+msgid "queued command:"
+msgstr "команда поставлена в очередь:"
+
+#: src/nexus_sync/cli/__main__.py:124
+msgid "command:"
+msgstr "команда:"
+
+#: src/nexus_sync/cli/__main__.py:141
+msgid "result:"
+msgstr "результат:"
+
+#: src/nexus_sync/cli/__main__.py:146
+msgid "stdout:"
+msgstr ""
+
+#: src/nexus_sync/cli/__main__.py:148
+msgid "stderr:"
+msgstr ""
+
+#: src/nexus_sync/cli/__main__.py:155
+msgid "CLI for nexus-sync server API"
+msgstr "CLI для API сервера nexus-sync"
+
+#: src/nexus_sync/cli/__main__.py:158
+msgid "nexus-sync server URL"
+msgstr "URL сервера nexus-sync"
+
+#: src/nexus_sync/cli/__main__.py:160
+msgid "list clients"
+msgstr "показать список клиентов"
+
+#: src/nexus_sync/cli/__main__.py:161
+msgid "print raw JSON response"
+msgstr "вывести необработанный JSON-ответ"
+
+#: src/nexus_sync/cli/__main__.py:164
+msgid "show client info or queue a command"
+msgstr "показать информацию о клиенте или поставить команду в очередь"
+
+#: src/nexus_sync/cli/__main__.py:165
+msgid "client id"
+msgstr "идентификатор клиента"
+
+#: src/nexus_sync/cli/__main__.py:167
+msgid "queue a command for this client"
+msgstr "поставить команду в очередь для этого клиента"
+
+#: src/nexus_sync/cli/__main__.py:173
+msgid "command timeout in seconds for --run-command"
+msgstr "таймаут команды в секундах для --run-command"
+
+#: src/nexus_sync/cli/__main__.py:176
+msgid "show command execution info"
+msgstr "показать информацию о выполнении команды"
+
+#: src/nexus_sync/cli/__main__.py:177
+msgid "command id"
+msgstr "идентификатор команды"
+
+#: src/nexus_sync/cli/__main__.py:228
+#, python-brace-format
+msgid "nexus-cli error: {error}"
+msgstr "ошибка nexus-cli: {error}"
+
+#: src/nexus_sync/cli/__main__.py:253
+msgid "id"
+msgstr ""
+
+#: src/nexus_sync/cli/__main__.py:254
+msgid "client_id"
+msgstr "id клиента"
+
+#: src/nexus_sync/cli/__main__.py:255
+msgid "hostname"
+msgstr "имя хоста"
+
+#: src/nexus_sync/cli/__main__.py:256
+msgid "platform"
+msgstr "платформа"
+
+#: src/nexus_sync/cli/__main__.py:257
+msgid "version"
+msgstr "версия"
+
+#: src/nexus_sync/cli/__main__.py:258
+msgid "created_at"
+msgstr "создано"
+
+#: src/nexus_sync/cli/__main__.py:259
+msgid "last_seen_at"
+msgstr "последняя активность"
+
+#: src/nexus_sync/cli/__main__.py:260
+msgid "kind"
+msgstr "тип"
+
+#: src/nexus_sync/cli/__main__.py:261
+msgid "name"
+msgstr "имя"
+
+#: src/nexus_sync/cli/__main__.py:262
+msgid "status"
+msgstr "статус"
+
+#: src/nexus_sync/cli/__main__.py:263
+msgid "timeout_seconds"
+msgstr "таймаут (сек)"
+
+#: src/nexus_sync/cli/__main__.py:264
+msgid "delivered_at"
+msgstr "доставлено"
+
+#: src/nexus_sync/cli/__main__.py:265
+msgid "finished_at"
+msgstr "завершено"
+
+#: src/nexus_sync/cli/__main__.py:266
+msgid "return_code"
+msgstr "код возврата"
+
+#: src/nexus_sync/cli/__main__.py:283
+msgid "<empty>"
+msgstr "<пусто>"
diff --git a/tests/conftest.py b/tests/conftest.py
new file mode 100644
index 0000000..a1748aa
--- /dev/null
+++ b/tests/conftest.py
@@ -0,0 +1,13 @@
+import pytest
+
+
+@pytest.fixture(autouse=True)
+def _force_source_locale(monkeypatch: pytest.MonkeyPatch) -> None:
+    """Keep CLI output in the source language during tests.
+
+    ``nexus-cli`` activates a locale from ``NEXUS_SYNC_LANG`` / the system
+    locale, so assertions on English output would break on a machine whose
+    locale is, e.g., Russian. Pinning to a language with no catalog makes
+    gettext fall back to the source strings regardless of the dev's environment.
+    """
+    monkeypatch.setenv("NEXUS_SYNC_LANG", "en")