Multilanguage support

If sending a newsletter with translated nodes there are important conditions:
- Both newsletter nodes should share the same
-- newsletter term: Choosing different terms will lead to an error
-- publishing state: Any node with unpublished state will lead to ignore

Checking a setup with
- default language english
- first translation language german

Having a bunch of subscribers.
- users with none / default and alternative languge preference
- anon subscriber with none / default and alternative languge preference
(That's 6 cases!)

Test (single mail) sending cases:
- Newsletter source != default language
-- user with source language
-- user with translated language
-- anon subscriber with source language
-- anon subscriber with translated language
-- user without preference
-- anon subscriber without preference
- Newsletter source = default language
-- user with source language
-- user with translated language
-- anon subscriber with source language
-- anon subscriber with translated language
-- user without preference
-- anon subscriber without preference
All tests passed.

I wonder if there's a lovely simpletest integration to automate such tests...

Spool deletion also tested, works cleanly.

Also real sending does the language switch.
But the reported send counts are wrong:
Translation source node reports ALL recipients while all translation nodes report ZERO sends - although the correct sending separation happened.
See separate issue for this:
#875672: Wrong "Sent" numbers on "Created issues" list

Since fixing the counter needs an API rewrite we'll release it with wrong counters by intention.

Note i've committed some improvements while testing, see attachment.

Possibly this should be checked, if port is needed to D7.

Here some additional references to multilingual issues
#1070730: Replace i18n_selection_mode with new concept
#1177640: Add own explicit base url to send reliably
#1305318: Translation of newsletter categories

Oops, correct tagging.