If the languages is already in the database then use the guid that exists. If not then add the language and use whatever guid is created after adding it to the db.
Fo existing languages, the guid is already known because they exist in the sql setup script that inserts the row for the language, so they are the same on all installations. However if you add your own languages and others add their own languages (even if you both add the same one) they will get different guids on the way into the db.
Hope it helps,
Joe