How to translate Organic Groups Menu?
Good question (+1) but be aware that this is a known issue about the OG Menu module.

There are a few issues related to it, and issue 2348821 contains a great summary of these (related) issues. It also includes some patches (the patch in #9 seems to be the recommended one to consider).

The issue is sill in status "needs review", so maybe you want to consider participating in that review? I.e. check if that patch works for you, and update the issue with your review results?