Some ways could be that when people migrate they bring their language to other places. Like, chain migration (when are person follows their family or people from their same origin to a new place) they come in usually large numbers and make the language more popular in that area. Like the US immigrants from Mexico and other Spanish speaking countries moved here they brought Spanish which is now spoken by many people. Also, migration could have a role in how some languages are lost because some migrants adjust to fit into the place they moved to. I hope this helps a little...