Depends. Generally English, unless they are “directed” to one specific person like the professor who’s gonna grade it. But even then I might go with English anyway.
Oh I also use my language when I’m leaving an important warning to myself in a config file, like “this is needed because X! don’t touch it! If you touch it do Y!”
The code for the peeler is stale, it stopped working three carrot seasons ago, but no one wants to rewrite the PeelerBladeRdge class.