Hello @Florian, I was just wondering perhaps you could make this legacy feature a toggle-able option? (which defaults to hide language code (suitable for most average users), but toggle-able to allow advanced users like @poster to define their own language code), this can probably make everyone happy.
Another big reason is:
The Comments frame and Unsynced Lyrics frame both have the same frame structure (as can be seen and compared in both two referenced ID3v2 standards links), but the Comments frame hides the language code, whereas the Unsynced Lyrics frame doesn't. This seems kinda inconsistent for two frames with the same structure, so adding the previosly mentioned toggle-able option can solve that problem.
My another related post: About the Language Code for Comment and Unsynced Lyrics