Translatable fields UI

Over the past two weeks at the core internationalization virtual sprint, we've been working on translatable fields. This would allow any core field attached to a node, user, taxonomy term etc. to be multilingual.

Core already allows for translating one entire node into another node, but you can't for example have translatable user profile fields, or taxonomy descriptions - and we're hoping this patch would be an enabler for that (and also make node translation more flexible).

However, there's no UI yet, and it might be a tricky problem to solve, hence this post.

We've got a couple of very basic ideas which I'll try to summarize here. I was going to do rough wireframes for these, but it seems better to get the post up first and add those later - and all the ideas are quite vague at the moment.

*1 Different tabs for different languages.

node/1/edit/en - editing the node in English (only field in English are editable and saved)
node/1/edit/fr - editing the node in French (only fields in French are editable and saved)
node/1/edit/fr/en - translation English to French - either the fields are pre-populated with their English values, or we show them side by side.

Pros: would allow for quite granular permissions like "only the Russian translator is allowed to translate pages into Russian"
Cons: Currently no way to show all values for a field at the same time (although that could be added possibly)

*2 Vertical tabs (or similar) on the node form itself.

In this case you'd have a text field with

[English]
[French]
[German]
Down one side, and when clicking on a language, javascript allows you to edit the field in that language.

Pros: you can easily switch back and forth between different language versions of your content without losing form values
Cons: Permissions would be trickier (although possibly not impossible).

*3 Language drop down
This isn't really a translation issue as such, but it's an issue we should deal with - when entering content, how do I choose which language the initial fields are in? Does it use site default language, do I choose it for every field at once, or per field (for an example, switch on locale module and allow nodes to have languages - you'll see a language drop down per node there).

So, we need a UI that will actually work for translators, and it also needs to be something which we can re-use across at least core object types like nodes, users and taxonomy terms. These 2-3 ideas are obviously very rough sketches at the moment, so, ideas welcome!!