What is due diligence when translating a paper? EDIT:  Part of the community has decided on a less catchy and more representative title
than "Socially acceptable plagiarism (with regards to translation)".  Let's run with that for a while. GRP.10.27 END EDIT.
The catchy part of the title refers to reusing words or ideas without permission, but without
'rocking the boat'.  In mathematics, this is often done by proper attribution of the 
source, by naming the author/speaker, or providing other clues that the phrasing one has
just used is not original.
If this were a discussion forum, I would invite answers that lists various forms of socially
acceptable plagiarism; instead I prefer brief mention of such to the comments.  This post is about the propriety and etiquette involving translations.
My situation is that I want to devote time to producing an English translation of some papers in German, and make the results available.  Assume (although I may end up doing something different) that I place my efforts in a PDF and put it on some web page for public Internet access.

Primary question: What trouble do I get into by doing that?

Answers to this may depend on what things I did wrong, especially if I use certain words,
phrases, images, parts of images, without crediting the source or asking for permission.  So let's add to the context that the papers are from before 1980, the authors are likely unresponsive, and the source journals are likely discontinued, although online
access to the sources are not freely available.  I will list the original sources in the PDF, but the following question arises:

What trouble may happen if I do not contact some representative and ask for permission to use old material?

For material published in 1980 and afterwards, there is less of an excuse not to seek such permission or rights, and I would accept anecdotes that might lend guidance and are not obvious applications of common sense, but I am interested in the amount of effort someone in academics expends in order to reuse published material, especially in translated form.
I know of a few examples in book publishing where more effort is made in getting such
permission, however that appears more expensive than I feel the current scenario merits.  This leads to:

What is "due diligence" in producing such a translation?
  Does it matter if the translation is provided gratis or for a fee?

I don't expect to sell access to the translation; it is my intent to make it freely available to any individual researcher.  If someone else wants to put it in a book and sell that
book, however, perhaps I could grant them such rights in exchange for a small monetary (or caffeine-ary) consideration.
Finally, let's assume I provide my own translation, except that for some small
sections (possibly in a different language) I use someone else's translation of the same or related source.  Let's say that I am concerned especially about a fragment that
is (roughly) three paragraphs or about 200 words long.  The answer to the following questions may be length dependent: if so, consider that I also have a 20 word fragment that is
of concern.

How do I attribute this fragment?  Do I use a footnote, or mention it in a preface?  What is due diligence for making sure I can use this fragment?

This question is barely suitable for MathOverflow;  I ask it here because the papers and output are mathematical, and the conventions in mathematics and mathematical publishing may not
be addressed were I to ask these questions elsewhere.  If they are addressed elsewhere, please provide a pointer to such material.
Also of interest, although I do not need the answers here, is if I have the same situation as above, except that I provide an interpretation (which is laced with my own perspective) rather than a translation (which attempts semantic fidelity and objectivity with respect to the 
source paper).  The answers may stay the same, but it feels like a different situation to me.
(A similar situation is mentioned in Mathematical etiquette: Rephrasing / restructuring a work, limited release (with attribution) acceptable? , which has some useful advice, although it does not involve natural language translation.)
Gerhard "Yes, It's About Jacobsthal's Function" Paseman, 2011.10.26
 A: I want to acknowledge the contributions others have made; I choose the answer format to help with emphasis.
I should have mentioned copyright issues in the question, as that is the major consideration when asking about trouble; thanks to
Michael Greinecker for pointing this out and to Henry Cohn for
expanding upon this.  (This teaches me to preview questions on 
meta.mathoverflow where being, shall we say, less thoughtful carries less stigma or less embarrassment.)  Since this handles the primary
question, I will accept one of their answers if a) they submit an
answer with that content and b) no one else gives a more thorough answer regarding due diligence.
Much as I liked my original title, I thank darij grinberg for changing it
to reflect a key issue of the series of questions. (I do have mixed
feelings, grumble, grumble.  At least I read the bit in the FAQ about
collaborative editing before posting.)  Although I am still interested in hearing of other examples of socially acceptable plagiarism, I agree with darij that this question is more about due diligence with respect to translations.
I am glad for Gerald Edgar's contribution; I am hoping to see more
anecdotes like those, and I should have said so more loudly.  I will
wait at least three days and then accept his answer if I do not see one I like even better.
I also thank David Speyer and KConrad for their remarks.  I have not yet decided, but their comments sway me towards writing an expository article which contains my intepretation of the papers of interest, and adds some original material.  That would force me to do more summaries and cutting, but that may be a good thing.
Based on the number of views and votes, and the fact that the question is still open, I thank the community for tolerating this kind of question.  I hope more good answers will appear so that this can serve future readers.
Gerhard "Can't Wait For Oscar Night" Paseman, 2011.10.28
A: Comments.  For my book Classics on Fractals I published translations of various relevant papers.  I wrote to the copyright owners (such as learned societies who published the journals) for permission to do this.  (This was back in the Olden Days, 20 years ago, when email was not as common as today.)  My publisher (Addison-Wesley) gave me the wording to use in making the requests.  In a few cases, the publishing organization asked for some payment in return for permission, or for a copy of the published book.  In a few cases the original copyrighting organization no longer existed but some successor organization provided permission.  For 3 (out of 20) papers I got no response, or did not find anyone to ask.  But I kept the documentation of what I had done in case they would surface later (none has in 20 years).
