Hi Andi, sorry for not replying sooner, been busy.
Shame it takes so much effort to get the localized SM. I will note that someone (Andreas Kohl?) uploaded a de localized SM to Hobbes and there is a localized Lightning at
http://mozilla.akohl.net/file/lightning-3.3.6.os2-mitropa.xpi, includes German, Hungarian, Russian, Italian and Polish.
This is well as there never was an official SM 2.28 release so some hacking is required to localize.
As for Lightning, I don't really use it so am not an expert but have been told that one problem is that Google keeps changing its API which breaks the synchronization and it's been suggested that I don't even distribute .gdata-provider.xpi but as it shows up in my build...
I did have some problems with SM 2.21 not loading some pages after some uptime which was very irritating and I would have to reboot to fix. Never figured it out here and it would actually affect all browsers once it started. That problem seems to be gone with 2.28.
Given a shortage of time, you might want to wait for 2.35, which hasn't even been released on the tier 1 platforms (it's quite late) but I'm hoping to release once Bitwise finishes with the 38ESR work (most of the code is shared).
Hopefully someone else can make Lightning recommendations.
One thing I've been curious about, I take it that de_AT is Austrian German. Is it much different then German German or Swiss German? I'd prefer en_CA but put up with en_US as it is close enough, especially if I use a Canadian dictionary as Americans spell lots of words differently.