Updated article: Choosing a Language Tag

A new section “Decision 6: Extension subtags” was added to refer to the new u extension, registered by the Unicode Consortium to add information about language or locale behavior.

This change produced editorial changes to the second subsection following the new text, now title “Grandfathered tags”.

The section “Further Reading” was also overhauled.

Translators should consider retranslating the affected sections.