Currently, it is possible for someone to define a different locale, which is not in the allowed locales list. It can lead to certain issues. eg. On concept forms, the locales listed are the ones defined in the allowed locales list global property. This means that, if the default locale is not defined in that list, it will not show. The default locale should always show up on the available locales on the concept form.
1. The propose concept feature creates synonyms in the default locale, meaning if the default locale is not in the allowed locales list, these synonyms won't be seen on the concept form since the default locale tabs won't be visible.
2. If i create/edit a concept where the allowed locales list is blank, a list containing only the default locale is returned, on the concept form we will only see the default locale meaning the new concept name(s) will get created in the default locale. After saving and the user later sets the allowed locales and excludes the default locale, it means the name(s) previously added while the allowed locales list was empty won't be shown on the concept form since the default locale tabs won't be visible.