In order to customize your query to return results only from site collections created in a specific language you have the following option:

<QueryText>contentclass:sts_site language:en-us</QueryText>

By specifying the language as a keyword property in the query you will show the results only from en-us site collections.

In SharePoint 2010 the same result is achieved differently because search.asmx is taking the language attribute found in the QueryText xml element and adds it as a Culture property of the Query object:

<QueryText Language=”en-us”>contentclass:sts_site</QueryText>

Additional information regarding Culture property and REST API:

http://msdn.microsoft.com/en-us/library/office/microsoft.office.server.search.query.queryproperties.culture(v=office.14).aspx
http://blogs.msdn.com/b/nadeemis/archive/2012/08/24/sharepoint-2013-search-rest-api.aspx
https://msdn.microsoft.com/en-us/library/office/jj163876.aspx