Language resources without valid configuration should be shown with brackets in "Add" dialogue

Currently in a default translate5 instance some language resources can be selected for adding them, for example DeepL, are addable as language resource, even if they are not properly. Others are correct in this regard (like PangeaMT).

This must be changed so that all language resources that are not properly configured are shown in brackets. Those language resources should then not throw messages about missing configs (please see TRANSLATE-2061 for information about this). If a language resource is properly configured, it will also not throw error messages and will be shown without square brackets in the GUI.