For those who do not know it it lets you activate translation and then a small symbol appears next to every word on any given page letting you translate what you can actually see. That way, after installing the correct language files, you can translate anything that is missing with a single click.
Not the same thing you described, but just making sure that you know this plugin: GitHub - pkp/translator: PKP Translator plugin for OMP 1.1.1+ and OJS 3.0+