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" />
It's not a good practice. Most probably the search engines will ignore them or even penalize you for spam.
Anyway, why would you have multiple description meta on the same page? I don't really think that your page will be in 3 languages at the same time.
I suppose you will have some language selection function or automatic selection based on user's browser language settings. Then you should output the appropriate description based on the selected language.
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.