mediawiki: Allow setting site language code

The site language code should be set in the configuration file /etc/mediawiki/FreedomBoxSettings.php. It has two purposes:

• This specifies which language your page content is in (by default).
• This also specifies the language for user interface messages. Anonymous visitor will see the interface in this language. Logged-in users and set their interface language in preference.

After updating this value in settings file, messages cache must be purged with command echo 'MessageCache::singleton()->clear()' | php maintenance/eval.php. Users must be warned that changing the language will purge any message customizations.