Yeah in swedish leaving them off isn't working. The diacritics aren't for accentuation, they are distinct letters. An ö is as different from o as u and e are.

Same in Hungarian. One funny example is "főkábel" (fő+kábel, main cable) vs. "fókabél" (fóka+bél, seal intestine). Still, the intended meaning is almost always easy to guess.

Hungarian is also redundant enough that you can even replace all vowels with just one and still be understandable. Most of the meaning is carried by the consonants. E.g. "Szia én vagyok Péter, hogy vagy?" -> Szii, in vigyik Pitir, higy vigy. (sounds obviously wrong, but very understandable) Retaining the vowels but collapsing all consonants to one would be more destructive to the meaning.

Uh, what do you mean? Leaving them off and letting the reader guess the intended word works pretty well in practice and is what many organisations in Sweden resorted to when getting a domain name (ex: riksgälden, åhlens, företagarna).

It works fine in practice, even if pedantic people will always correct you when you use a instead of å.

And 1337-speak was never readable to anyone in English, since numb3rs ar3 d1st1nc7 fr0m l3773r5.