Rewrite of translation handling

-New languages can be added using XML only
-TEXT XML files can have translations for any number of languages
-English is used when strings aren't present for current language
-TEXT strings can be added with English as the only language
-Empty strings are ignored (same as not present)
-TEXT strings no longer cares for order of languages
-Selected language is stored in ISO 639-2 code to avoid depending on language index

Details on adding new languages can be read in comments in newly added XML\GameInfo\CIV4LanguageInfos.xml
Text XML cleanup (removed empty strings)