Windows XP is written in specific language versions, therefore one
cannot change it from one language version to another. You'll need
to purchase a "Full Version" of Windows XP in the language version
you want, reformat your drive and install it "clean".
Or if you have an available empty drive or partition, you can install
another "Full Version" of Windows XP in a different language version
and create a dual-boot configuration.
Nope. You must purchase the full version (unless you have a qualifying
product in the same language - a Polish version of 2000 cannot be used as a
qualifier for an English XP, for example) in each language you require.
There is no conversion.