Since it was written in the wiki, i was wondering where i can find that .lang file. Couldn't find it and so was not capable to add custom lootbags that have nice names as description without that "name.xxxx.name" pre-&suffixes.

Thx in advance

using LootBags-1.11.2-2.4.3.jar

since i am also using llibrary for jurassicraft i hope both .lang files are not affecting each other.

You make one yourself with all the unlocalized names of the new bags and their translations, package that into a resource pack, and load it.
https://minecraft.gamepedia.com/Tutorials/Creating_a_resource_pack will get you about 90% of the way there. It doesn't say clearly, but you can merge/replace .lang files like you can everything else. The file structure for my .lang files is assets/lootbags/lang/en_us.lang

can't do it :D language-packs are not present anymore in the newest minecraft versions. So i was previously doing already "The file structure for my .lang files is assets/lootbags/lang/en_us.lang" but still it doesn't work

I tested it in 1.12.2 before posting that, I know it still works fine. Make sure the pack.mcmeta file has the correct pack_format number in it for 1.11 and that the .lang file has things listed the correct way, they should look like:
name.lootbags_lootbag_Derp.name=Derp Bag