Manual:$wgOverrideUcfirstCharacters

Language, regional and character encoding settings: $wgOverrideUcfirstCharacters
List of Unicode characters for which capitalization is overridden in Language::ucfirst.
Introduced in version:	1.34.0 (Gerrit change 502546; git #d46835ef)
Removed in version:	Still in use
Allowed values:	(array)
Default value:	`[]`
Other settings: Alphabetical \| By function

Details

The characters should be represented as char_to_convert => conversion_override.