The public property $LOCAL_LANG_charset within AbstractPlugin (formerly known as pi_base) has been removed.
The property was used to hold information which labels, coming from TypoScript that override Language files, need to be converted to UTF-8.
Accessing the property within own extensions has no effect anymore.
Installations with extensions that provide plugins derived from AbstractPlugin and use this property.
No migration needed. Make sure that all external TypoScript configuration files are stored with UTF-8 character set.