Function resolvePreferredLanguages

Resolves the preferred-language option to an ordered list of BCP 47 tags, most-preferred first.

• a string becomes a single-element list
• an array is used as-is (already ordered)
• null falls back to the platform's navigator.languages order — null means "use the platform preference", not "no preference"
• an empty list (empty array, or navigator.languages unavailable) means no preference: keep all languages