its the default WordPress behavior, user profile settings override global site settings.
if translation is nit available , then original plugin language is used, US English (unless plugin author does some weirdness)
you can alao be interested in this plugin
https://projectdmc.org/plugins/preferred-languages/
it also can be merged with WordPress core in the next releases
Hi @delanet,
As mentioned by @fierevere, for logged in users of your site, their preferred language will always be used. If they are not logged in or their preferred language is not supported, the site language is used instead.
We do not offer a means of changing this within the plugin options. I’ll note it for consideration, but in our opinion at this stage, this is better managed at a WordPress level as suggested.