Integration with i18n/translation requires changes in content_taxonomy

I've been trying to get content_taxonomy to play with i18n's i18n-taxonomy and translation modules. Ideally any integration problems should be solved in i18n so that other modules do not need to be explicitly i18n-aware. I tried this approach however and it is very difficult. i18n works by rewriting SQL statements to append extra WHERE clauses for language codes when it deems appropriate. When translating nodes that have content_taxonomy fields it is all but impossible to detect when to add these clauses and the SQL required in some cases is a bit hacky.

The main problem arises when a parent term is chosen for a content_taxonomy field. This parent term id (tid) needs to be translated into a corresponding translated parent tid before querying child nodes.

The patch I've attached adds integration with i18n on the content_taxonomy side. I've tested all code paths with i18n installed but not without.

I've also submitted a proposed patch to content.module to fix a problem with pre-selecting option list values.

This patch does not fix synchronisation (an experimental module in i18n). I hope to look at that too.