On Android, Circuit will automatically use your phone language if it is supported. If it is not, it will default to English. There is no way to change language used in Circuit other than to change your phone language.
On iOS, you can change the language on a per-app basis. For that, please follow these instructions:
Go to your phone Settings.
Find Circuit and tap on it.
Select a language of your choice in 'Preferred Language' setting.