To switch Osmo interface messages to a different language, you need to change the language setting on your device.
Here are the instructions from the Apple website:
If you're using an Amazon Fire tablet, open your Settings, and search for 'language' (options you need to select depend on your tablet model).
After you do that, open any Osmo app, and you will be prompted to select your preferred language for that Osmo app. If you want, you can change the system language back to what you had it as before and keep Osmo in the new language (it will prompt you to ask the next time it starts up).