Normalise string (remove diacritics, accents and co)

You can use a custom font with all the possible characters, you can use a standard font, or dor things like day of week, use your own strings.

for example:

var now=Time.now();
var clockTime = Calendar.info(now, Time.FORMAT_SHORT);

Then with an arr you can use and an array like

var days=["","Sun","Mon","Tue","Wed","Thu","Fri","Sat"];

days[clockTime.day_of_week]

gives you a DOW as a 3 char str.

You also could use strings for each language with only the characters you want.

I wouldn't walk the string and replaces characters

My watchface supports displaying the city from a weather API and also user provided strings - I need something to "clean" those strings because I need a custom font (I do display some texts rotated) which can not support all characters...

I do use a custom script that processes the weather api response and returns a minimal response to my watchface, so I can clean the city name in my service, but I need the same for user provided texts as well...

The best and most efficient thing then is to just have all the characters you may need in your custom font.

Impossible because this increases the memory footprint way too much... Still thanks for your suggestions.

walking the stings and replacing characters could be fairly inefficient so watch out for that.

A different approach that I saw used in an app on the app store is to send the string to a server and receive back an image for display. 

The app used this for notifications in character sets that Garmin won’t or can’t or just doesn’t support. But the same approach would work for basically any utf string as the server can have full fonts (and can generate rotated images).

That's even more inefficient than walking the string.  You got comm involved, and with the app being a watch face, could be done at most every 5 minutes.

True, but as part of the use case is a weather api... 

double the comm (or more), depending how many stings need to be converted.