I am sorry for misunderstanding. Yes SEO can only be 1 language. Frankly I am not sure how browser will handle different encoding if this is part of URL.
You can copy some foreign languages text from yahoo. co. jp/ (Japanese), tw. yahoo. com/ (Traditional Chinese), ru. yahoo. com/ (Russian) and input it to SEO keyword to see the result. In fact, abantecart use utf-8 and it should support all the foreign languages.
I have tried to input non-english text in the keyword and there is no error until now. Both IE and chrome can handle it.
Hope that abantecart can support real multi-language SEO. I have seem other cart who support real multi-language SEO before.