In the plugins $GLOBALS['TSFE']->config['config']['language']; is used to detect the language. This sets the BE language! This is wrong when used to send it to Google as TYPO3 doesn't use ISO names but it's own encoding. Use t3lib_cs->isoArray for a mapping (or use $GLOBALS['TSFE']->config['config']['htmlTag_langKey']).
Sidenote: I'm surprised that a FE plugin uses $LANG as localization module. The class is meant for the BE and there are other interfaces in TSFE and pibase for he FE.
In the plugins $GLOBALS['TSFE']->config['config']['language']; is used to detect the language. This sets the BE language! This is wrong when used to send it to Google as TYPO3 doesn't use ISO names but it's own encoding. Use t3lib_cs->isoArray for a mapping (or use $GLOBALS['TSFE']->config['config']['htmlTag_langKey']).
Sidenote: I'm surprised that a FE plugin uses $LANG as localization module. The class is meant for the BE and there are other interfaces in TSFE and pibase for he FE.