#5158 move language selection to the top

[Federico Bruni]

I've been thinking for some time that language selection should be moved to the top, for the following reasons:

1. It's the place where most people expect the language selection to be. There are obvious reasons for this.
2. Add to previous argument the fact that links in online manuals are nice links without extension and the browser always serves the file matching the browser language setting (issue [#2273]). So if you happen to use a browser in a language different from the language you'd like to read, you'll need to scroll down each new page to select the right language in the footer.