2
Short version:
1) Nope
2) Yup
3) Dunno
4) Yup
5) Don't think so (could use one, too)
Long version:
1) Documentation is not very detailed. So far, we have the wiki... people are on the task of documenting XOOPS, but it's a big task
2) Content is a bit hard to do. I think someone managed to get some modules working with multi-langual contents. If you just want the menu items and other things, which are the same every time you open the page, then it's just a matter of language constants. Check the language/[english/german/russian] folder to see, what is there and what isn't
3) I'm not familiar with Russian characters
4) In System Admin -> Blocks, you can set exactly where the login box is or isn't shown. Keep in mind, though, that XOOPS is "meant" to be a an open system, where login is easy etc. so the basic idea is to have the login block on the front page. But it's up to you of course.
5) I've found some things around on the forums. I'd say that the items in the system preferences can be accessed on all pages. Things such as server timezone, root URL, Site name etc. can be found on all pages. Apart from that, I think you'll need to be more specific to get an answer you can use regarding functions to use
A list of variables would deffo be useful.
GL with XOOPS