I had the same issue. I erroneously installed OJS with fr_FR locale (= French French!) which was available in 2.4.X before upgrade, but not yet for OJS 3.0. Later, I shifted to fr_CA (Canadian French), in addition to en_US
I did check Settings-->Web-site-->Languages, where I was able to get rid of the void fr_FR locale.
Now I see only en_US and fr_CA in my settings dashboard,
but I still get three fields wherever else I go.
Is it possible that since some fields related to the void fr_FR locale are containing data somewhere in my configurations, the field still shows up everywhere else?
Not a big issue, but quite annoying.