Language
English and Czech, switched anywhere.
The app is in English and Czech. The EN / CS switch is in the top right of your profile (and of the first screens when you sign up), and changes everything immediately.
Screenshot to comelanguage-switch.png
Which one you start in
A phone set to Czech starts in Czech; everything else starts in English. Once you've switched it yourself, your choice is remembered and the phone's setting is never used again.