Considering Guda goes through Orleans, Sumer, and a bunch of other countries, I think it's safe to say that their mystic code give them a translation worm of sorts. The servants around them can speak modern languages thanks to the grail, but for all we know they could be chatting in ancient welsh and Guda wouldn't know either. Which is really a long way to say, wouldn't it be cool if post-canon Guda called the local restaurant to order something the same way they'd talk to tamacat, only to get in response a very polite Mx, I'm sorry, but none of the staff speaks Hungarian, could you repeat that?
Alternatively you could headcanon that Guda does not have any sort of translation worm and just becomes increasingly good at bullshitting their way through thing. Anytime they speak to someone who isn’t a servant internally they’re just taking apart contextual clues/tone of voice/what little linguistic knowledge they’ve gathered trying to figure out what’s being told without having to ask Mash every single time. Guda doesn’t back away no matter how terrifying your threats are because they have no fucking idea what the fuck you’re saying.