Most frontends (I don't know any, really) doesn't allow you to set a default language for your post. For that reason, a lot of frontends allow you to post without any language selected, while defaulting to English. But that is not universally true. Photon recently started requiring language to be selected and started defaulting to Undetermined language. This now creates unnecessary friction when the community doesn't have it as allowed language and requires the user to select allowed language for every single post.
It really is a simple change on your end that will take you seconds, but can save a lot of time in the long run and some frustration for people that want to post in your communities.
On lemmy-ui frontend you can change it by going to community settings and under languages you can select multiple ones by holding Ctrl key.
P.S. This community is a great example of one that doesn't allow Undetermined language.
Actually, if a users doesn't select a language, the client is supposed to submit the post without any language selected (not even undetermined). THE SERVER then defaults the post to one of the languages allowed in the community.
This is probably how Photon has worked until now, since it obviously never got an error no matter where you posted, even though it didn't have a language selector at all.
But since photon now has a language selector, it is now defaulting to making posts with "undetermined" unless the user changes it, instead of null. If it was using an actual null value then the posts would be correctly defaulting to something valid for the community.
Source: Thunder ran into the same issue when language selection was added to posting and commenting.
@Xylight@lemdro.id might want to experiment with that a bit.