Update locale_get_display_x() return types

[cfahner]

For issue #12942 I updated the signatures of the following functions:

• locale_get_display_language() and Locale::getDisplayLanguage()
• locale_get_display_name() and Locale::getDisplayName()
• locale_get_display_region() and Locale::getDisplayRegion()
• locale_get_display_script() and Locale::getDisplayScript()
• locale_get_display_variant() and Locale::getDisplayVariant()

The _language() and _name() variants never seem to return an empty string (at least I could not find any case for which they did), so I left them at non-empty-string.

The _region() function returns an empty string when you give it a language code without a region.

When you pass in a very long string these methods all return false (C source code).

I added my quick testing script as an attachment. This is my first PR here so let me know if there are any other requirement.

test.zip

[ondrejmirtes]

Thank you!