How to get current site (locale) api / plugin
I don't know if this was the problem you were having but I was not including the site's first URL segment like /en/ (in your example) with the controller action URL. So you'd want to post to /en/actions/pluginHandle/controllerClass/actionMethod to get the English site back when using Craft::$app-&gt;getSites()-&gt;getCurrentSite() in your plugin.

I posted a more thorough answer to the same problem in another ticket:
https://craftcms.stackexchange.com/a/33941/4433