I think a lot of people take throwback projects and retro computing too far by rejecting everything modern. What's wrong with allowing some anachronisms for things like high DPI, internationalization, security, accessibility?
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.