i18n: Move locale select logic in home to i18n.py.

Part of #16094.
Moved the language selection preference logic from home.py to a new
function in i18n.py to avoid repetition in analytics views and home
views.

zerver/lib/home.py

@@ -10,6 +10,7 @@ from two_factor.utils import default_device
 
 from zerver.lib.events import do_events_register
 from zerver.lib.i18n import (
+    get_and_set_request_language,
     get_language_list,
     get_language_list_for_templates,
     get_language_name,
@@ -136,17 +137,11 @@ def build_page_params_for_home_page_load(
 
     furthest_read_time = get_furthest_read_time(user_profile)
 
-    # We pick a language for the user as follows:
+    request_language = get_and_set_request_language(
-    # * First priority is the language in the URL, for debugging.
+        request,
-    # * If not in the URL, we use the language from the user's settings.
+        register_ret['default_language'],
-    request_language = translation.get_language_from_path(request.path_info)
+        translation.get_language_from_path(request.path_info)
-    if request_language is None:
+    )
-        request_language = register_ret['default_language']
-    translation.activate(request_language)
-
-    # We also save the language to the user's session, so that
-    # something reasonable will happen in logged-in portico pages.
-    request.session[translation.LANGUAGE_SESSION_KEY] = translation.get_language()
 
     two_fa_enabled = (
         settings.TWO_FACTOR_AUTHENTICATION_ENABLED and user_profile is not None

zerver/lib/i18n.py

@@ -5,10 +5,12 @@ import operator
 import os
 from functools import lru_cache
 from itertools import zip_longest
-from typing import Any, Dict, List
+from typing import Any, Dict, List, Optional
 
 import orjson
 from django.conf import settings
+from django.http import HttpRequest
+from django.utils import translation
 
 
 @lru_cache()
@@ -84,3 +86,22 @@ def get_language_translation_data(language: str) -> Dict[str, str]:
     except FileNotFoundError:
         print(f'Translation for {language} not found at {path}')
         return {}
+
+def get_and_set_request_language(
+    request: HttpRequest,
+    user_configured_language: str,
+    testing_url_language: Optional[str]=None
+) -> str:
+    # We pick a language for the user as follows:
+    # * First priority is the language in the URL, for debugging.
+    # * If not in the URL, we use the language from the user's settings.
+    request_language = testing_url_language
+    if request_language is None:
+        request_language = user_configured_language
+    translation.activate(request_language)
+
+    # We also save the language to the user's session, so that
+    # something reasonable will happen in logged-in portico pages.
+    request.session[translation.LANGUAGE_SESSION_KEY] = translation.get_language()
+
+    return request_language