I love when I read these comments ahah, it makes me laugh that people come for the NSFW and then stay for the story and the humor.
As I said in another comment:
look at the username logged on the PC. Many apps during installation might say something like “Good morning [username], we’re installing updates…” etc. It’s the same mechanism: there’s a function in the engine’s code (Ren’Py + Python) that lets you read that name, but obviously it never leaves your PC.
For the language instead, instead of taking the logged username, I take the system language and the time zone. For example, if the language is French and the time zone matches France, then 99% you’re in France.
So I created quite a few combinations: it doesn’t always get it right, sometimes I make mistakes (especially in South America), but most of the time probability works in my favor.