I am looking for a possiblity or plugin that redirects the user only if he hits the homepage to the appropriate language subdirectory. If the user goes to the right language specific page, e.g. domain.com/fr, the plugin shouldn't force a redirect and respect the users choiche!
If the user selects another language the selection should be remembered for the next visit for the whole site. I do have the theme's language selector in the header and the translation is done with langify.
Does anybody have any recommendation or know a possibility to implement this?
Here is one tool for you: Geo Redirect. It has all features that you requested.
It can auto direct customers to correct URLs based on their geolocations. You can choose to display a customisable permission popup requesting for redirection. The tool will remember the visitor's selection for future redirection. It also provides the advanced referral whitelisting that disables geo redirection, which is useful for allowing visitors to switch sites without being forced to redirect.
Hope it helps!