MongoDB aggregation: Replacing sub-document with a value out from it - arrays

Question

Let's say I have a collection of documents in the following format:
{
// some fields
"name" : "some name",
"specs" : [
{
"key" : {
"en" : "English key name",
"xx" : "Other key name",
},
"value" : {
"en" : "English value",
"xx" : "Other value",
}
},
{
"key2" : {
"en" : "English key name2",
"xx" : "Other key name2",
},
"value2" : {
"en" : "English value2",
"xx" : "Other value2",
}
},
//and some more sub-documents
],
}
I'm trying to query it from the database to get it in the following format:
{
"name" : "some name",
"specs" : [
{
"key" : "English key name",
"value" : "English value",
},
{
"key2" : "English key name2",
"value2" : "English value2",
},
//and some more sub-documents
],
}
How can it be done, if it is possible at all?
Background
I'm making a software which must be available in multiple languages, and I think current document schema is most suitable for this (if you've got better ideas for the schema I'd like to see them).
To minimize amount of data queried from the database, I'm trying to select the data only in one language. And moreover I want to minimize nesting of structures in the code, so I'm searching a way to somehow select a value out from a sub-document and replace the sub-document.
I've tried a lot of ways writing such query. Here's the one, but it doesn't work as I expect it to:
db.software.aggregate({
$project : {
"name" : true,
"specs" : {
"key" : "$specs.key.en",
"value" : "$specs.value.en"
}
}
});
It transforms a key into an array of all "key.en" fields within specs field. May there be a way to reference a current array element inside "specs" instead of the whole specs array?