I can see where you're coming from but consider it from the opposite perspective when you want to translate a page. There's two ways to go...

• First, now you are on the edit page but haven't added a translation, I would need to separate the translatable data and the non-translated data. It could be odd to see the separation of this data as it's not always very organizable.
• Second, I could allow things to automatically be created but then it is often the case users aren't always 100% clear on what is being edited and add incorrect data. At least this way it forces users though a single standard system on all editable content when these pre-requisites aren't met.

Having said all that though, in the case of a birthday, it's just 1 extra click. I wouldn't be too worried about it, click it and add/change the birthday.

Can we just have a 2nd button on that page which says: Edit in language which you already have?

At the moment, there is only one option. Would be nice to have "Add translation" + "Just edit". :P