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})