Multiple charsets and Collations for a Multinational Database
If the data is the same (same tables/columns) and the only change is that now there is the possibility of more languages being used, I'd say go for UTF-8. Force it as the default charset on all the table definitions (new ones at least), and set it in your config to be used for client connections that don't specify a charset.

You have not specified what DB type you are using (MySQL, SQL Server…etc) so I am trying to be as general as can be :)