Should be self-explanatory, so i just post an Image:
https://ibb.co/mR3B5b

--Edit by Thommy101--
Mekanism doesn't use "Mekanism-specific" localisation keys, causing issues with other mods using the same localisation keys.
Happens mainly with guis and liquids.

Essentially the same as #4698 but worse

Same for portable teleporters.

Are you using any custom language packs? As that value ("Are you sure you want to delete this?") isn't part of our default localisation.

No, i'm not. I use PureBDCraft as Resource Pack for Skins, other than that, nothing.

What can i do to find out which mod is fucking up something there?

Aah... I could see what's happening.. There seems to be a localisation key conflict. We (sadly) do not use a Mekanism specific localisation key.
It seems another mod is also using gui.delete, with the value you're seeing. You could try to add a en_us.lang file in a resourcepack with the following in it (just a plain text file) as a temporary solution:

gui.delete=Delete

Thank you! :)

Somehow i couldn't find that issue...