GNU Unifont should really be the default font.
I have brought this up a few times in IRC, it is non-obvious that the user needs to have the foresight to select a font that includes the characters needed for their dialect before changing the language, and that not doing so may have results that range from complete success, to a completely unreadable and largely unusable mess based on rolling the dice on the combination of their chosen dialect and Rockbox port (the default font for cabbiev2 isn't implemented uniformly across the different ports).
But, fixing this brings its own set of challenges to the table that need discussion.
[Saint]