language/usage.rst needs update for 3.2

[nickvergessen]

The logic was moved to phpbb\language\language instead of phpbb\user

@MateBartus did anything else change? https://github.com/phpbb/documentation/pull/20/files#diff-095e33362bf3f1179099f1e83357dfde

[CHItA]

Actually nothing should have changed, the add_lang() logic is a bit different, but if someone uses the user class then it should not matter anyway (in the rst it is in this way).

[Nicofuma]

For 3.2:

• twig lang() function: {{ lang('my_key') }}
• language service
• BC with /phpbb/user

[CHItA]

I'm not sure if it should go into the doc is the change of the user::lang member var. The way core files used it in some places to check whether the user object is inited, was to use if (empty($user->lang)) which is no longer works, and user::is_setup() method should be used instead. IMO this is only relevant if there is any possibility that extensions using something like this.

[iMattPro]

Handled by #38