Do the characters ever mention the location being Japan or speaking Japanese directly? Is it localized at all? Like the English version of Bandori Garupa will just say stuff like “Weird that they’re speaking English, when we’re speaking Japanese.” or “I can’t wait to go back home to Tokyo.” that sort of thing. I know Kanade vaguely mentioned “characters” when talking about Mafuyu using the name “Yuki”.
It's stated that it's set in Japan and they speak Japanese, however I recall it being mentioned that they speak English like literally one time. I'll look into this one further because I cannot remember where this is mentioned at all let alone if I didn't make it up in my head just now.
It’s nothing definitive, but Episode 3 of the Kamiyama High Cheering Squad event, KAITO said this during the Liu Bei discussion
It’s likely been translated this way in order to explain the joke to non-Japanese speakers while still keeping the story flow, but it stood out to me because it just seems so… clunky? It seems to imply that they do speak English, or at least, not Japanese, because why else would KAITO specify?
This is likely not the example you’re looking for, but it does imply inconsistency with the language they use. (Maybe it’s Japanese everywhere else, and the Wonderland SEKAI uses English)
The Wonderlands SEKAI can't speak English, though, because Tsukasa routinely remarks about how his English is bad (from scaring the little girl during their trip to America and afterwards one of the Chibi dialogues being along the lines of 'Stars around the globe are taking to the worldwide stage! WonderlandsxShowtime is no exception!' 'What should we do about it?' 'How about writing our shows in English?' 'Wouldn't that confuse the little ones?' 'Urk... You have a point' 'What if we used simple phrases to make it a learning opportunity?' to which Tsukasa proudly praises the idea before saying that he really needs to brush up on his English).
There is, I believe, also a couple one-off Nightchord jokes about how Mafuyu speaks pretty decent English, but I can't honestly remember where that would have shown up.
I think, what's slightly more likely and would make sense in universe and outside, other than it just being a simplistic way of translating (cucumber) is that some of the Virtual Singers, like Luka, have English voicebanks?
In Japan, though, names are often specified as being written with certain characters! Since they don't have a technical alphabet like English does, it is quite common for the characters written to have different pronunciations or their own meanings (as people don't name themselves after objects)! "This is my name, and these are the characters you use to write it-" is decently common in Japanese media (at the very least).
