I have Steam for Mac and every time the app updates, the language of the entire thing is set back to Spanish, my system language, and only changes after a million tries of setting the preferences. Opting out of the beta didn't help either.
Have you submitted a bug report? This definitely seems like a bug, and ultimately a fix will be better than any workaround people here can provide.– pushashaAug 17, 2016 at 2:54
Thanks for the suggestion. I was discouraged from submitting because of the noncommunication.– inakilbssAug 17, 2016 at 9:28
At least this question is the very first result for “steam "system language"” on DuckDuckGo. Maybe that'll bring some people who can add knowledge to it.– inakilbssAug 17, 2016 at 12:07
There's already a thread on the Steam Community Forums: http://steamcommunity.com/discussions/forum/2/360671352681228444/?ctp=2
Look like it's a problem that affects everyone with the latest versions of both OS-X (El Capitan) and Steam (Aug. 16 2016 patch).