Using a non-English locale would always fallback to the customized English one, the most prominent example being the trust level names.
English:
Any other language (using spanish here):
This occurred when the devforum went down for maintenance to update to the latest version. Unless the previous behavior was the actual bug, devforum doesn’t use the same naming scheme for trust levels as the default ones, and should probably be looked into.
I’m not sure this is new. This is because we only override the trust level names in English locales. We don’t use the standard trust level names it seems like. (new user → basic user → member → regular → leader)
I turned on a setting earlier today for serving the forum in system locale if the user has not explicitly set one. Did you have the forum on Spanish locale before today?
Yes, before I would use devforum in Spanish, although I will admit I had it in English for a while before the maintenance as I needed to get the name of a setting in English, and just never bothered to change it. The trust level names always used the overwritten ones (English), the forum not falling back to the default locale only started happening after the maintenance. So I don’t know if the falling back was the intentional behavior since technically if you change it discourse can’t know if it has the same meaning, or if that was actually a bug since you set a different locale and therefore discourse should use those language’s strings.
I changed it back afterwards because I submitted a PR for a plugin a while back and knew the plugin would also be updated here, therefore I changed the language back to Spanish to see the results. (satisfied 
)
I understand that updating the set of trust level names to each language supported by discourse would not only be challenging, but low priority, but maybe translating them into X amount of popular languages on the forum could probably be looked into, or whatever is best.
I don’t think we ever intended it to fall back to the changed English terms. The current behavior is what I thought was the case even before the update.
I can look into if I can programmatically set the trust level names in all locales to be the same as the English ones for consistency with the pinned posts