The language in your app is based on the Language Settings in your phone.
Go to Settings --> General --> Language and Region to change it from there.
For Android users:
If your phone allows you to create a list of preferred languages, then the change of language in Clue should be immediate. You should check the Languages setting in your phone Settings, and see if this is possible. If it is, then make sure that your preferred language for Clue is in that list, and it’s in the first position.
In some cases (for instance, Xiaomi phones) it’s not possible to save a list of preferred languages - you can only select one language at a time. So, after changing the phone language, you will need to delete and reinstall Clue in order to make the change effective in the app. Please save a backup file of your data before doing this (the backup function is in the Menu under Settings), and be aware that, if you delete and reinstall the app, the list of your custom tags will be reset. The custom tags tracked in the past will still be saved in your tracking history, but you will need to type them from scratch in the tags list.
For iOS users:
If your phone is in a language other than English, you may change the app's language by going to Settings -> Clue (among the installed apps) -> Preferred language.