A while ago I took an existing 3x8 font, converted it by hand for use with AdaFruit's graphics library and subsequently modified it to use ISO 8859-2 characters (in a presumably innocent coincidence this popped up in the Adafruit GFX Library a few months later). Anyway, I recently received a request to perform the same modification to the standard 5x8 fonts and only recently got round to looking at this. As a quick reminder, ISO 8859-2 covers ~128 extra characters for alphabets used by Central and Eastern and Southern European languages - these characters look like this:
After a little bit of experimentation I realised that the extra 2 columns aren't really all that useful for adding the extra ligatures (čarky, hačky, umlauts etc) - we're really constrained by vertical space. Many of the existing letters need to be modified or reworked entirely, since they consume the almost entire 8 rows and we need 2 rows for some ligatures. For example Ä is currently implemented as the following:
0x7D, 0x12, 0x11, 0x12, 0x7D
Which looks like this:
This is visually a little confusing, but more importantly we cannot really re-use it for ISO 8859-2 since some of the ligatures we need to add to the "A" require at least two rows. Instead of having an "A" which jumps around depending on the ligature, I've created a single A for when a ligature is used and left the ungarnished original letter alone.
Just as another example of why this can be tricky the existing ä looks really weird to me, the umlauts are skewed off to the left and look like they're joined to the letter itself.
I've moved this up into a central position which is the same on all letters involving umlauts. This is purely based on personal taste, but I think it looks better - below is the original style compared to my modified version:
There are similar considerations in some of the other letters that are left as an exercise for the reader - see if you can devise a neat system to fit all the letters below into 3x8 grid in a way that is consistent and legible, it's pretty tricky. I've made an initial stab at this (see GitHub gist below), but after revisiting this I've realised how flakey and error-prone this process of creating fonts is.