How to replicate:
1) Set Minecraft to German.
2) Place Coarse Dirt anywhere.
3) Middle Click the Coarse Dirt (it will show the name at the hotbar in english)
4) Scroll to a different slot and back to coarse dirt (it will now show "Grobe Erde" which should also happen on middle click.

What should happen:
Both middle click and scroll should show "Grobe Erde" in German.

What happens:
Middle Click shows English not German Blockname.

Edit:
When coarse dirt is not selected in the hotbar but is in there and a middleclick is perfomed it shows "Grobe Erde" another middle click and it shows "coarse dirt".

Screenshot after middle click:

Screenshot when scrolling to the item: