Is it valid to define multiple meta description with different languages? Is this valid?
<meta name="Description" lang="en_US" content="Something in here" /> <meta name="Description" lang="pt-BR" content="Algo aqui" /> <meta name="Description" lang="fo-BA" content="Foo bar" />
Valid? Yes. There are no real restrictions on what you can have in the way of meta data.
The problem with not having a clear spec is that different consumers will consume them in different ways.