Whether the route is straight UI controller or API controller, the logic for determining the call's locale should be the same:

locale = user-default-locale
locale = request-header-locale if !locale
locale = server-default-locale if !locale
locale = english if !locale

Note too that this locale value needs to propagate to all subsequent backend services calls that are made (eg. the locale should be passed in the request header to pulp, candlepin, async jobs, etc.). This already happens for katello services but I am unsure if same is true for foreman.