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.
In the next version or the one after that, I believe it will default to your browser/system language. The undetermined language thing isn't actually good for anyone.
Least of all, people finding communities in their tongue.
If Sync ever updates, I hope it adds language to the post form.
this is only for setting the default user language during registration based on the browsers accept language headers.