Untranslated fields

well what I was thinking is when we do submit, we can handle the empty fields in validation function, and try to remove them or make them null if that is the right thing to do. I don't know if there is a better approach.

I just looked at the way node module is handling empty fields, you'll get an empty array of this field (e.g. ['field_name']['lang_code'] is empty).

This issue also affects multi-value fields, I tried to validate all fields before submitting them, and passing them to setTranslation(). This issue doesn't appear in node's submit (without translation, e.g. add new article), because node's submit validate all fields and remove the empty ones, haven't found it in node's module to use here, so I tried to remove them manually.

Updated patch against the current head, this is an important one, could we have it in next release?

I didn't get what you mean by field-specific way, but as I can tell from testing that neither the default attach_form_validate() nor field_attach_validate() can address this problem because of one thing, that they rely on 'language' index inside entity which we have it always set to original language (because we have field based translation, not entity based), that's why the default validate fails to get the language and iterate through it.

I'll try to use field_attach_validate hook to do our custom validation and iterate through all possible languages inside form values.

Here is a rework of the previous patch, can't get the field_attach_validate to do the job in hook like:

translation_field_attach_validate($entity_type, $entity) {
...
}

so I put the logic inside the validation callback.

I've tried almost all the possible ways to do so, and checked how node & fields ui do the validation also, as I said in the previous comment our problem is language inside entity only set to source language that's why default validation fails to detect translated fields.