Consolidate Gitlab::I18n setup

What does this MR do and why?

Initialize FastGettext in a single initializer.

Moving setup logic into Gitlab::I18n makes it easier to extend it later. See #19484 and WIP !74391.

Screenshots or screen recordings

Language preference

How to set up and validate locally

In Rails console:

Gitlab.I18n.locale # => "en"

Gitlab.I18n.locale = "de"

_(" or ") => # " oder "

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.