Ensure there can't be duplicate culture codes in languages dropdown

[bergmania]

Fixes #11427

[Zeegaan]

Stylecop suggests using a method body

public IDictionary<string, string> GetAllCultures() =>
            // get cultures - new-ing instances to get proper display name,
            // in the current culture, and not the cached one
            // (see notes in Language class about culture info names)
            // TODO: Fix this requirement, see https://github.com/umbraco/Umbraco-CMS/issues/3623
            CultureInfo.GetCultures(CultureTypes.AllCultures)
                .Select(x=>x.Name)
                .Distinct()
                .Where(x => !x.IsNullOrWhiteSpace())
                .Select(x => new CultureInfo(x)) // important!
                .OrderBy(x => x.EnglishName)
                .ToDictionary(x => x.Name, x => x.EnglishName);

[bergmania]

I saw that, but really didn't like it with the big comment 🙈

[Zeegaan]

Found a minor thing, i tested this and it works 👍

Rest of the code LGTM 💪