I wonder if tools similar to gettext exist for manuals? FWIW, I have never seen any. Something based on wdiff might be better than straight diff.
E.g., does `info' supports several translated versions of a manual? There is no special support for translations in the Texinfo system. In fact, there is little support for languages other than English (although some non-English manuals have been written). This is of course a very serious deficiency, but it's not easy to remedy, and I regretfully have not had time to do much about it. If yes, where should they be installed and how should they be named? There is a French translation of the Info manual. It's named info-fr.texi and info can access it just like any other manual. FWIW. A better approach would probably be $(infodir)/fr/info.texi. But this actually requires changing the code to figure out where to look under what circumstances, and thus hasn't been done. (I see nothing about this in the Texinfo manual, but maybe there are plans for this.) It's come up many times. Wish I had better news to report.