I was never officially taught Sütterlin, but through family and other circumstances I can read it fairly well after a bit of a "warm-up" period.

What's interesting is that it's pretty much impossible for me to read if used for a non-German language. Sütterlin for English text? My brain cannot parse this at all - the script automatically flips my brain to German!

That makes a lot of sense, given that in the Kurrent era it was actually considered proper to use a "Romance" (and hence "modern-looking") script even for non-Germanic loanwords in German text, mirroring the Fraktur/Antiqua distinction in print typesetting!

In a way, this could also be compared to the present-day use of katakana for loanwords and hiragana for native text in Japanese (which ironically only crystallised as a universal convention after WWII).

My (German) grandmother used to write me letters in English using this script. I didn't find it too difficult to read, probably because I understood the context (names of other family members, questions about my day-to-day activities, updates on her life in Germany).