1 - Check if your language pack uses utf8 or required character set (iso-8859-x)
2 - make sure your db & all of your tables collation is utf8 general or unicode.
3 - If the issue is still unresolved, open locales.php and set :

$force_io_charset_if_accepted = 'utf-8'

and

$db_config['connection_charset'] = 'utf8'

or replace utf8 parts with custom charset with corresponding charset if you insist on using them. (iso-xxxx-xxx)