You can add the necessary language by adding the appropriate line with language data to "languages.php" file (wp-content/plugins/multilanguage/include/languages.php).
Follow the next steps:
1. Find the wp-content/plugins/multilanguage/include/languages.php using the FTP software.
2. Open the languages.php file in a text editor.
3. Add a language following the example of others from the list.
4. Save changes.
5. Add the corresponding flag icon image (png, 16x11px) to the 'images/flags/' folder using the FTP software.
When done, update the plugin settings page and you'll see the added language in the languages list.
In order to find the correct language name and language locale please visit the following page: https://en.wikipedia.org/wiki/List_of_ISO_639-1_codes
This article related to Multilanguage by BestWebSoft plugin
Additional resources: Multilanguage - User Guide
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