Fix: Slovenian language code is sl_SI

htdocs/langs/ar_AR/languages.lang

htdocs/langs/ca_ES/languages.lang

htdocs/langs/da_DA/languages.lang

htdocs/langs/de_AT/languages.lang

htdocs/langs/de_DE/languages.lang

htdocs/langs/el_GR/languages.lang

htdocs/langs/en_US/languages.lang

htdocs/langs/es_AR/languages.lang

htdocs/langs/es_ES/languages.lang

htdocs/langs/fi_FI/languages.lang

htdocs/langs/fr_FR/languages.lang

htdocs/langs/hu_HU/languages.lang

htdocs/langs/is_IS/languages.lang

htdocs/langs/it_IT/languages.lang

htdocs/langs/ja_JP/languages.lang

htdocs/langs/nb_NO/languages.lang

htdocs/langs/nl_NL/languages.lang

htdocs/langs/pl_PL/languages.lang

htdocs/langs/pt_PT/languages.lang

htdocs/langs/ro_RO/languages.lang