Andreas has recently mentioned that SerenityOS is meant to be an English-language OS. (Might have been on Discord or a Q&A video, not sure.)

I believe his rationale is that other languages are generally awkward at talking about technical stuff, and SerenityOS is addressed at technical users anyway. In my own experience as a French native speaker, I do tend to prefer all my software to be in English rather than getting the awkwardly-translated-in-French version.

As far as accessibility goes, like anything in the project, it's up to someone passionate to step up and work on it.