That looks like a missing language key, not an activation button. Go to Studio - Polyglott - Keys and search for _sys_status_pending. I guess that there is no entry in the field. So fill the text in.
I compared that to my 12.1 site. Hmm, I'm not sure. The switcher is a form element and processed by call to a handler, depending on its status on/off. It seems that something went wrong and some code got mixed up here. It still looks like a language key to me.
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