fix(i18n): use babel.locale_selector instead of babel.localeselector for Flask-Babel 4.0.0

- Flask-Babel 4.0.0 changed the API
- @babel.localeselector -> @babel.locale_selector
- Remove redundant request import in context processor
- All tests still pass

app.py

@@ -26,7 +26,7 @@ app.config['LANGUAGES'] = {
 # Configure translation directory
 app.config['BABEL_TRANSLATION_DIRECTORIES'] = 'translations'
 
-@babel.localeselector
+@babel.locale_selector
 def get_locale():
     # Try to get language from URL parameter
     lang = request.args.get('lang')
@@ -41,7 +41,6 @@ def get_locale():
 # Context processor to make language and config available in templates
 @app.context_processor
 def inject_global_variables():
-    from flask import request
     lang = get_locale()
     return {
         'language': lang,