We should either bundle or generate on-demand first-letters-XX.ser files.
ICUCollation itself, as well as PHP's collation, accepts language strings like 'sv' or 'pl' just fine. However, lack of corresponding first-letters file causes a cryptic exception.
If the 'root' file is copied, sorting works correctly for given language, only the headings are incorrect (default, not taking letters with diacritics like Ø or Ą into account).
The files can probably depends on Unicode tailoring data to add additional letters to create subheading for in categories. http://developer.mimer.com/charts/tailorings.htm looks like a good starting point.