added module based on #github/iionly/Elgg-1.9_German_Language_Pack.
Although the original translation by iionly is intended for Elgg-1.9 it is still very useful for 1.8 AND it has AGPL compatible license (Dual licensed->MIT).
The version for 1.8 is only released under GPLv2 which is unfortunately incompatible with the AGPL of this project.

Using submodule does not seem reasonable at the moment as the version differences will make contributions to the original repository improbable.