The biggest challenge I've had when writing multilingual user interfaces aren't lacking a way to translate, but just practical issues like dynamic string construction or where the structure of the UI somehow doesn't work in another language, or when a given string is used in multiple parts of the app in the English version, but the non-English versions need different strings in different places[0], or just where an English single word translates into a whole sentence (or vice versa).
[0] For example some languages don't have a commonly used word that means "limb" - i.e. arm _or_ leg. A bit niche, but if you're doing something medical-related it can cause issues.