I quite like the Tolino and Kobo range of products but currently don’t have a Kobo at hand.
Since I just created complete new German hyphenation patterns and dictionaries for the Tolinos, addressing some of the problems @Leonatus mentions, I wonder if someone could test if those might also work on the Kobos.
Apart from the Hunspell- (Hyphen-)compatible formats (hyph_de.dic), Tolinos also seem to use the Hyphenator Javascript (de.js pattern file) which I also created.
Please refer to my (German-language) post
Tolino - Deutsche Silbentrennung, an archive containing both files is attached there. It uses 3/3 hyphenation, both to address some problems with attached punctuation and to reduce the number of "2-letter hyphenations" which can slow down the reading flow.
If this works out for use on Kobo devices, I’d be happy if you’d include it in your installation software. The files contain English-language comments, and are based on TeX’s
dehyph-exptl hyphenation (a 500,000+ word corpus), with additions to the word list, and recompiled patterns plus appropriate conversion to Hyphen and Hyphenator formats, with manual changes for Tolino quirks.