From my own translation attempts with Polylang, you have to translate plugin named vouge-pro
and edit the syntax there
Edit
My bad as I misunderstood what you are trying to achieve. So, I will re-iterate of what I now think you want to achieve:
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You want to have frontend of a site available in English and Spanish. This works for you, however
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Now you realised, that Spanish speaking admins may want to see the backend of a site in Spanish
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I also assume, that English speakers want also to see the backend of a site in English
If that is true, you need to install another plugin to be able to achieve this.
Quick search found Simple Admin Language Change to be able to do that.
In my own case, as a Czech speaker, if I want to present a site in more languages – in my case mostly in Czech and English, I solve it by simply changing WordPress site language to English and translate only frontend (end-user facing) strings