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.
One of the things that really make sense but might be overkill is to have integration with any of the open source Large Language Models. The client could wouldn’t have to ask the user or rely or system settings, and autodetect the language. Depending on the content, it would even make sense to have some override system (e.g, I am not fluent in Spanish, but I would be interested in seeing football news from La Liga directly in Spanish without having to wait for the UK newspapers to pick it up)