In the case the watch uses English despite that your selected language in System settings is non-English, you need to switch the language to English, restart the watch (may not be necessary, but it is safer), then changing the language to your liking, and then you'll be prompted to install it with Garmin Express.
The extra step of changing the language to English first (despite the screens already being in English), is not always apparent, and it may confuse users not familiar with it, but in some cases you can't install the foreign language without doing it first.
