The font textures from german "Umlaute" (äÄöÖüÜß...) and maybe also other characters are not loadings from resource packs (I tested the same font textures in older Minecraft Versions (1.8.9). there its working correctly). Its not working since at least Minecraft 1.15. I also testet at least 5 different Ressource Packs with custom fonts. Non of those are working (The "Umlaute") but the letters are in the Texture Pack Font File.