The Germans do not like ligatures across compound words, and that is much harder to do in an automated way (not in fonts themselves at least).
That's a good point, but it's a slightly different issue from prohibiting some ligatures altogether: in German ligatures should be disabled depending on context, and that applies to all ligatures that may be present in the font; while in Turkish some specific ligatures should never occur at all, but the others are free to go. While we're on the subject, and since Thomas mentioned Reclam, I seem to remember that dtv, another paperback publisher, uses a font with the relatively rare ligature ft; I don't have any book by them handy at the moment, but I assume that they do apply the same rule as with the other ligatures and don't set them across compounds. That's what I meant by font-dependent ligatures; maybe I should have written typeface rather than font. Clearly you don't want to implement the prohibition of ligatures across compounds at the font level. Arthur (but I have the TeXbook on my way to BachoTeX. It does use ligatures)