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!”
So glad I study CS and our teachers are just “here’s a LaTeX template for the thesis, it will do the formatting for you”. You can always use word and follow the formatting guidelines yourself, but that would be stupid