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:[]

See T219279.

Details edit

The characters should be represented as char_to_convert => conversion_override.