To be sure, they were invented in California by a German, a drygoods merchant named Levi Strauss, but the word itself has been around a long time. It comes from Gênes, which is a French name for Genoa, the Italian city where the jeans material (denim) was first manufactured. The word denim itself is an Anglicization of de Nîmes, which refers to the French city where the jeans material became popular. But it wasn't until 1850 that the real Cowboy pants were invented, by the aforementioned Levi Strauss.
In the US the word "jeans" was originally singular. Consider this quote from History of Menard and Mason Counties, Illinois, by Robert Miller, published in 1879:
"Flannel and linsey were woven for the wear of women and children, while jeans was woven for the men."
Note the conjugation of the verb "be" after jeans. I think it did not start to become plural till later, as anglophones were simply accustomed to thinking of words ending in S as plural entities.
Later, when France and Germany borrowed the fashion of the denim jeans from the US, they borrowed the pronunciation as well, so they end up with jeans (sounds like "genes"). But they did not adopt the pluralization (except apparently in Braunschweig).