First of all, I know long pressing on a char like 'a' gives me choices e.g. 'aä..' so the special char (umlaut) 'ä' can be selected. But this works only for me for the "international"-style keyboard ...
I am running CM10.1 (Andromadus flavor, based on Android 4.2) on a HTC Desire Z with built-in hardware keyboard. To input keys that are not present on the keyboard, there is a symbol key that pops up ...