This way of thinking is why I, as a blind developer, need to deal with so many horribly inaccessible tools, I guess.
If you think that only the consumer-focused part of your tools needs to be accessible, you're wrong.
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.