Billy is set to Danish by default, but you can easily change the language of the interface to English.
How to change the language
To change the language from Danish to English:
Go to Settings (the gear icon in the bottom left corner).
Click Profile at the top.
Change your language from Danish to English in the Your language field.
Note: Make sure you have filled out all information under Settings → Organisation. Otherwise, the system may have trouble switching the interface language to English.