Add language autocompletion

[ghost]

Closes #284

[boyska]

what happens if navitgator.language is empty, or if its value cannot be found in the language list?

except for this corner cases, seems good

[ghost]

it just disappears the placeholder string

[boyska]

leonaard:

it just disappears the placeholder string

yeah, but I guess that if the user submit the volume, it could be with
an empty language, or with an invalid langcode

[ghost]

the browser doesnt submit the form if there is no valid language, try it

[boyska]

the browser doesnt submit the form if there is no valid language, try it

I tried and it does. Since there is no practical way of forcing an
invalid language in firefox, I manually opened the firebug console and
typed document.getElementById('language').value = 'zz';
I sumbitted it, and it is added. I think that this should be prevented
client-side (server-side we should accept any two-letter code).

[boyska]

Oh, wait; it used to have the bug I reported, but now I see that everything is working fine, o0

merging