I'm not sure how long it took, but for a while the instance had crashed. For some reason the Lemmy photo upload service took up 150GB of free space and hence the server crashed because of it. I'm not sure about the possibility of recurrence as I don't know the exact reason.
For this reason, you can follow our Mastodon account, which I have just opened, to follow or report possible crashes.
https://mastodon.world/@lemmynsfw
Update :
I can change it now. It seems that I should choose either "Undetermined" or "English" in the Languages option. So, I choose only "Undetermined"
At default state, Undetermined and English already checked :
At least I noticed that when the community has no icon or wants to upload an icon for the first time. Before that, I never did anything in the Languages option and was always successful 'til now