How do I update MongoDB document fields only if they don't exist?

You can add a query to your update statement:

db.foo.update({'title.de': {$exists : false}}, {$set: {'title.de': ''}})

Update

For your modified question my solution looks like this - would that work for you? (If not, why?)

db.foo.update({site_id: 'xxx', 'title.de': {$exists : false}}, {$set: {'title.de': ''}, {multi: true})
db.foo.update({site_id: 'xxx', 'content.de': {$exists : false}}, {$set: {'content.de': ''}}, {multi: true})

there is a update field operator $setOnInsert which meets your requirement. Please read the documents here: https://docs.mongodb.com/manual/reference/operator/update/setOnInsert/#up._S_setOnInsert

I have a solution for one particular case, but may be it helps someone.

My case was: Update several fields, among which was a field that had to be updated only once (lets call it "Date_of_first_update").

> db.test.find();
{ "_id" : ObjectId("57f298fdeb30478a033c70e4"), "a" : "1", "b" : "2" }

First update:

> db.test.updateOne({ "_id" : ObjectId("57f298fdeb30478a033c70e4")}, 
  {$set: {a: 100, b: 200 }, $min : {'Date_of_first_update' : (new Date())  }});

Result: 'a', 'b' updated, 'Date_of_first_update' is set.

{ "_id" : ObjectId("57f298fdeb30478a033c70e4"), "a" : 100, "b" : 200, "Date_of_first_update" : ISODate("2016-10-03T**17:47:43**.570Z") }

Second update:

> db.test.updateOne({ "_id" : ObjectId("57f298fdeb30478a033c70e4")}, 
  {$set: {a: 400, b: 800 }, $min : {'Date_of_first_update' : (new Date()) }});

Result: 'a', 'b' updated, 'Date_of_first_update' left unchanged, as I needed!!!

{ "_id" : ObjectId("57f298fdeb30478a033c70e4"), "a" : 400, "b" : 800, "Date_of_first_update" : ISODate("2016-10-03T**17:47:43**.570Z") }