March 2021
I managed to do it by deleting all the folders for languages other than english/the language I wanted in the "Locale" folder (backed it up first just in case). Done on Windows 10, hope this works for you too!
October 2021
just delete all other languages, works for me
July 2022
I tried all the advices mentioned above. Only this one worked for me. Awesome, thank you for this post.
February 2023
There is a quite simple and drastic fix.
Just delete all the languages beside the one you want from this folder:
C:\Program Files\EA Games\Spore\Data\Locale
June - last edited June
For everyone playing this game with the EA app (or, Xbox PC game pass), you'll need to change your Windows' locale to the one you want to play the game with. No need to add any startup option or delete/modify game files. Say if you want to play the game in Japanese, you'll need to change your "non-unicode program" language to Japanese, reboot your PC, and then launch Spore in Japanese.
See this link for how to change your Windows locale: https://thegeekpage.com/non-unicode-program/