On my (very old) Ericsson p800, the keyboard only had English keys, but software added the missing characters. An example for Swedish keys (some experimentation will likely find better ways): long-press o for 'ö', long-press a for 'ä', and long-press, extra-click a for 'å'. Does the software provide something like this?

It really is important that the special characters are available in some way, since without them many languages can not be written (the extra characters are often truly letters in their own right, not just variants of other letters).