No, it is not possible - at least yet.
There is a problem with characters encoding and different languages. Each language uses its own character encoding (like iso-8859-1 for English and Spanish, iso-8859-2 or windows-1250 for Czech, ...). If you select language for the slice, you also select (implicitly) the character encoding. If we allow users to use different language in the same slice, each item will be in different encoding, ... which means problems.