Aijin, I don't mean to correct Mcjoin01 here but is slightly off.
I imported a Japanese DSi and I sometimes play my Jap games on a DS lite as well.
1) Any DS games can be played on any DS system (excluding the new "3DS" or whatever since that is a completely diffent game system yada yada)
2) There are exceptions, which are:
-- two-three games that use camera made for the DSi (nothing to do with language)
--games that use extra atachments like "guitar hero" don't work on DSi (nothing to do with language)
--Downloadable games on DSi are region specific, but that shouldn't be an issue here anyways
The DS is very easy system to import for. You can even do download play with multiple systems! I say go ahead and buy it!
EDIT] The 3DS is very cool but they decided to make it region locked so that means importing is going to be a pain.

FYI
Edited: 2011-01-25, 5:04 am