Add Dutch locale label

[huulie]

1. Why is this change necessary?

The community skeleton can enable locales through the app_locales parameter. A companion community-skeleton PR adds Dutch (nl) translations and enables nl, but core-framework currently does not include nl in the locale label map returned by UVDeskService::getLocalesList().

When nl is active, UVDeskService::getLocales() attempts to read $localesList["nl"], which causes an undefined array key warning while rendering the sidebar language selector.

2. What does this change do, exactly?

Adds Dutch (nl) to the core-framework locale label map:

• nl is labeled with trans("Dutch").
• This matches the existing locale-list pattern used for English, French, German, Portuguese, and the other supported locale codes.
• It allows Dutch-enabled skeleton installations to render the sidebar language selector without failing.

3. Please link to the relevant issues (if any).

• Companion community-skeleton PR: Dutch translations community-skeleton#922