Revert "remove unused code -SvtLanguageTable::AddLanguageTag"

This reverts commit d812b784313ed85ae5085d54a3ae50aa064c053c.

I added that code in preparation of changes I'm doing.
remove SvtLanguageTable::AddLanguageTag from this list

... I'm about to use.

Change-Id: I4f665e14d0d2161085760a6b31f642e0a8a8345b