SQL: How can I get the value of an attribute in XML datatype?

Use XQuery:

declare @xml xml="<email>
  <account language="en" />
</email>"

select @xml.value('(/email/account/@language)[1]', 'nvarchar(max)')

declare @t table (m xml)

insert @t values 
    ('<email><account language="en" /></email>'), 
    ('<email><account language="fr" /></email>')

select m.value('(/email/account/@language)[1]', 'nvarchar(max)')
from @t

Output:

en
fr