There's no reason to manage multiple charsets. UTF-8 can encode every language under the sun, is supported by every browser that has been out for the past 15 years. Moving to UTF-8 would make everything simpler for both development and installation, and avoid nasty surprises for people who stick to the English version but need to display Euro symbol or other non-ASCII signs.
As such the MySQL tables should be also be built as UTF-8 (CREATE TABLE ... DEFAULT CHARSET=utf8) if this is not already the case.