Gramps has been translated in many languages, but there is always room for more! You can change Gramps to run in a variety of different languages. If Gramps is not available in your native language, you can translate it yourself, and have your translation included. Translating Gramps has all the details.
Furthermore, Gramps has the ability to provide culture/language depending relationships, dates in reports, as well as region specific date and calendar options.
If Gramps is available in your language, then perhaps the manual is not. You can also translate this, as explained in Translating the Gramps User manual.
The wiki could be translated, as explained in the Language policy.
What should I do if I see a translation issue? The easiest thing is to post a bug on the bug tracker or post a message on the gramps-devel mailing list. After doing that, you'll get introduced to the translators for your language and you can help them directly, or submit your own fixes. Alternatively, get involved by translating Gramps on Weblate.