The URL contains a language code, but that’s just an internal implementation detail.
Imagine a world in which Google Translate has one entry called “Latin” for anything using Latin script (Welsh, Irish, you name it) and another entry “Simplified Latin” for English. Just like that, translation “from TC to TC” (e.g., Cantonese to Hakka) is very much a thing!
A Mandarin speaker in Taiwan has a different accent and slightly different vocabulary than Mandarin speaker in Beijing, but neither of them would be able to converse with a Yue speaker any more than a Russian speaker would be able to converse with a Polish speaker.
Also, you’re very mistaken if you think all versions of English are mutually intelligible.
Second, the fact that they also speak Mandarin (because it was a mandatory lesson at school or they were otherwise forced by circumstances to learn it) does not mean there should be no support in Translate for their native language, which incidentally dwarfs many languages that Translate does support in number of speakers.
If you think that’s viable logic, you should try applying it to other languages with predominantly multi-language speakers. Start with Irish (98% of people in Ireland speak English, after all) and move on to Catalan, see where that gets you.