A few months ago, a friend of mine and I tried to ask O'Reilly Media whether it would be possible to publish the French Perl/TK O'Reilly book (out of print) for free on a developer community Web site. O'Reilly answered that they would not mind, but that we would first need to obtain the agreement from the author and from the translator. We haven't been able to get an answer from the translator so far. (It's a pity, since I know personally the translators of all the French O'Reilly books related to Perl, except for that particular book.)
So we haven't been successful yet, but it seems to show that it might be possible.
@ LanX: you might want to try to contact O'Reilly Media (especially if you happen to know how to contact the translator of the German PBP).