At many places in Core there have been charset fallbacks if forceCharset (or other charset definitions) have been set.
Until 4.5 iso-8859-1 has been default charset for TYPO3 but every set via forceCharset.
forceCharset has been removed at many places the default charset is used. this default charset has to be changed to be utf-8.
[TASK] Change various functions to use utf-8 by default
Many functions in TYPO3core expect a character set parameter to be
defined. Their default was iso-8859-1 in the past and is changed to