This does not happen with any other chromium based browser
My phone locale is set to Italian with English as second language. IP address is Italian. I’m not understanding why sites like f-droid, Google dev documentation, gitlab, are all defaulting to Chinese as I can’t read it…
Sometimes I can find a link to switch back to either English or Italian, but the next visit they revert to Chinese…
Have you checked the language settings of Firefox itself? (Assuming it just happens when using Firefox and not the native Browser)
on mobile the only setting available it’s for the UI language and there’s not the extensive language panel setting that is available on desktop. about:config is also missing
I think about:config is enabled on Nightly (possibly Beta as well?), and also the Fennec F-Droid build.
Then perhaps the “Translations” are set to automatic?