German from @virusperfect and Dutch from @rigrig. If anyone else wants to contribute, I've made it easier with the weblate instance.
Changelog
fad6a04 Added translation using Weblate (English (United Kingdom))
96abbdf Added translation using Weblate (German)
9a08a66 Deleted translation using Weblate (English (United Kingdom))
26a1f30 Fix initial language setting value
72a7759 Fix invite link when non-default language selected
e834445 Restructure language loading to support incomplete translations
887126f Translated using Weblate (English (United Kingdom))
91b2b44 Translated using Weblate (German)
14bb85f add dutch translation of admin pages
bf46c9f add dutch translation of emails
901abfb add translation from Weblate (German)
7c61392 add translation from Weblate (German)
882a346 fix language settings loading
b666547 make whole invite clickable to expand
e0a17c6 mention new weblate instance
27ac0bf merge german translation
7ce1b50 merge nl-nl with invite url fix
1aadd12 move validationStrings out of strings in lang/form
cd2ea2e reload email after lang is loaded
4485622 remove some comments
2eea836 translation from Weblate (French)
78b0e22 translation from Weblate (German)
8c276fa translation from Weblate (German)