[Baertram]

Perhaps we need to do the reloadui later then after all other stuff was loaded and you are read for the game.
The event is the event_player_activated.

Try this:

AdvancedLanguageSelector = {}
AdvancedLanguageSelector.name = "AdvancedLanguageSelector"
 
function AdvancedLanguageSelector:Initialize()
    EVENT_MANAGER:RegisterForEvent(AdvancedLanguageSelector.name, EVENT_PLAYER_ACTIVATED, AdvancedLanguageSelector.OnPlayerActivated)
end
 
function AdvancedLanguageSelector.OnPlayerActivated()
    EVENT_MANAGER:UnregisterForEvent(AdvancedLanguageSelector.name, EVENT_PLAYER_ACTIVATED)
    SetCVar("language.2","en")
    ReloadUI()
end
 
function AdvancedLanguageSelector.OnAddOnLoaded(event, addonName)
    if addonName == AdvancedLanguageSelector.name then
        AdvancedLanguageSelector:Initialize()
    end
end
 
EVENT_MANAGER:RegisterForEvent(AdvancedLanguageSelector.name, EVENT_ADD_ON_LOADED, AdvancedLanguageSelector.OnAddOnLoaded)