You might like to read this "how to" in the wiki.
I can tell you that Java Verified testing is cheaper if you put multiple languages in one JAR, because separate JARs get full testing each, whereas a combined, multilingual JAR gets only one full test pass (plus some additional language testing). Ovi submission does not require you to go through JV testing, but the testing they do internally is based on the JV process, so it is possible that a multilingual JAR might get through testing faster. But I don't really know what Nokia's internal process is.
So far as I know, you specify which region each JAR can be sold in. So you can distribute one or many. You can check this over at the Ovi discussion board.
Most game publishers prefer to deliver multilingual JARs, because the certification costs are lower.
On Nokia devices, you can use;
to determine the language of the device. You cannot guarantee this will be useful on some other manufacturers' devices.
If you add another language later, then either you distribute a separate JAR, or you have to update your existing JAR. If you can, release languages in groups, say English, French, Spanish, German as one, then Czech, Hungarian, Romanian as another, for example.
Hope that helps a little...