Code support for gettext has been added, but we're not using it.

And I guess there are some string in database that should be translated too (medium formats, releasegroup types, artist genders and types, relationships, ...)

As of today, MusicBrainz i18n supports:

• About 99% of coded messages now use gettext;
• Most strings in the database through “attributes” and “relationships” resources.

It doesn’t support (for now):

• Right-to-left scripts (see MBS-2598);
• Documentation (see MBS-1406).

It also has known issues about some yet messages that cannot be localized (see not closed Internationalization tickets).

To sum up, a few localized versions of the website are already available to visitors, but it is neither completely localizable nor usable by editors yet.