Parser::getFunctionLang() is identical to Parser::getTargetLanguage(). It's an artifact of an earlier stage in the parser. It should be deprecated and removed.
These functions have been identical since the creation of Parser::getTargetLanguage() in 7df3473cfea59df53debb7a9eefffed8a7f20fb3 (2012, MW 1.19).
See also T114640: make Parser::getTargetLanguage aware of multilingual wikis and T202481: Parser should have a msg() helper function so people don't localize messages improperly.
Change 836276 had a related patch set uploaded (by C. Scott Ananian; author: C. Scott Ananian):
[mediawiki/extensions/RSS@master] Replace Parser::getFunctionLang() with ::getTargetLanguage()
Change 836277 had a related patch set uploaded (by C. Scott Ananian; author: C. Scott Ananian):
[mediawiki/extensions/ParserFunctions@master] Replace Parser::getFunctionLang() with ::getTargetLanguage()
Change 836278 had a related patch set uploaded (by C. Scott Ananian; author: C. Scott Ananian):
[mediawiki/core@master] Deprecate Parser::getFunctionLang()
Change 836280 had a related patch set uploaded (by C. Scott Ananian; author: C. Scott Ananian):
[mediawiki/extensions/CollaborationKit@master] Replace Parser::getFunctionLang() with ::getTargetLanguage()
Change 836281 had a related patch set uploaded (by C. Scott Ananian; author: C. Scott Ananian):
[mediawiki/extensions/Contributors@master] Replace Parser::getFunctionLang() with ::getTargetLanguage()
Change 836282 had a related patch set uploaded (by C. Scott Ananian; author: C. Scott Ananian):
[mediawiki/extensions/EtherpadLite@master] Replace Parser::getFunctionLang() with ::getTargetLanguage()
Change 836283 had a related patch set uploaded (by C. Scott Ananian; author: C. Scott Ananian):
[mediawiki/extensions/HitCounters@master] Replace Parser::getFunctionLang() with ::getTargetLanguage()
Change 836285 had a related patch set uploaded (by C. Scott Ananian; author: C. Scott Ananian):
[mediawiki/extensions/WikiLovesMonuments@master] Replace Parser::getFunctionLang() with ::getTargetLanguage()
Change 836276 merged by jenkins-bot:
[mediawiki/extensions/RSS@master] Replace Parser::getFunctionLang() with ::getTargetLanguage()
Change 836281 merged by jenkins-bot:
[mediawiki/extensions/Contributors@master] Replace Parser::getFunctionLang() with ::getTargetLanguage()
Change 836282 merged by jenkins-bot:
[mediawiki/extensions/EtherpadLite@master] Replace Parser::getFunctionLang() with ::getTargetLanguage()
Change 836285 merged by jenkins-bot:
[mediawiki/extensions/WikiLovesMonuments@master] Replace Parser::getFunctionLang() with ::getTargetLanguage()
Change 836283 merged by jenkins-bot:
[mediawiki/extensions/HitCounters@master] Replace Parser::getFunctionLang() with ::getTargetLanguage()
Change 836277 merged by jenkins-bot:
[mediawiki/extensions/ParserFunctions@master] Replace Parser::getFunctionLang() with ::getTargetLanguage()
Change 836278 merged by jenkins-bot:
[mediawiki/core@master] Deprecate Parser::getFunctionLang()
Change 836280 merged by Jforrester:
[mediawiki/extensions/CollaborationKit@master] Replace Parser::getFunctionLang() with ::getTargetLanguage()