How can I change the language for the interface of SimPe? I know you can choose language in Settings, but it only changes the language of the game. I need to change the language that SimPE uses, though, for stuff like menues and sim personalities, etc. I think it tries to determine the language upon installation, but it didn't go well this time.

Try Extra -> Preferences -> SimPE Settings -> Default Language.

That's what I was trying to do, but that didn't change the interface language.

I know it's an old thread, but it's the first one in google when you search how to change language in simpe, so I'll post here.

I downloaded SimPE 0.73.44 and wanted it to be in English. Changing settings did nothing (same result as OP had).

Solution I found: I went where the SimPE is installed and removed the whole language folder (named "ru" in my case) and it worked. I don't know if it's the only way, couldn't think of any other. No problems so far, but I backed up the folder just in case.

By the way, I recommend anyone whose SimPE is in Russian, to change it to English, because the translation is screwed up. The EPs and SPs are mixed up in System Folders settings, where you choose paths to your add-ons. Instead of Seasons it says Happy Holiday, instead of Celebration it says Seasons, instead of H&M Fashion it says Celebration. I emailed Quaxi about this, so maybe it'll get fixed, but I have no idea.

ETA.
Quaxi's mailbox is returning an error, so my message wasn't delivered.
And I found another language folder, it's inside the "Plugins" subfolder. It translates plugins, like Object Workshop. I removed that, too ("ru" folder in my case). All seems to work fine.

Deleting the "Fr" folder works for french too.
Most tutorials online are in english; if the language of the program is in english too, it's easier to find what's what.

There's two language folders to remove, just like Lamare said.