MDL-74608 activities: a new option to force the activity language

For a long time, Moodle has had the feature to force the language
for a whole course. This change adds the same feature at activity
level.

The course-level feature was controlled by a capability
moodle/course:setforcedlanguage, and I decided to use the same
capability to control this feature. I think a new capability would be
overkill.

lib/classes/string_manager_standard.php

@@ -526,7 +526,7 @@ class core_string_manager_standard implements core_string_manager {
         $cachekey = 'list_'.$this->get_key_suffix();
         $cachedlist = $this->menucache->get($cachekey);
         if ($cachedlist !== false) {
-            // The cache content is invalid.
+            // The cache content is valid.
             if ($returnall or empty($this->translist)) {
                 return $cachedlist;
             }