If string profile field validation is set to any option excepting "Any chars", it is impossible to enter non-latin characters as it leads to error like

The field “Location” has invalid characters, only alphanumeric characters are allowed.

The problem is here: https://github.com/phpbb/phpbb/blob/develop-ascraeus/phpBB/phpbb/profilefields/type/type_string_common.php#L21-L25
Those regexes allow latin chars only.