Note that this method does not understand "similar" languages, so the following:
Locale::lookup(["en-US"], "en-GB", false);
Or:
Locale::lookup(["es-ES"], "es-CO", false);
Does not work as you would expect (empty result). To get a match in those cases you will have to use two letter language codes instead:
Locale::lookup(["en"], "en-GB", false);
Or:
Locale::lookup(["es"], "es-CO", false);
These do return 'en' and 'es' respectively.