You really can request that we add any book you want by importing the ISBN - there isn't any rule that they have to be English language. We have a huge backlog we are working our way through, though we are keeping on top of current requests too. There are a couple of problems with foreign language books - firstly that we don't necessarily know what the categories should be and secondly that because our data sources are mainly English language, it may be that we can't get the information. Once we have cleared the backlog and see all the requests left over for which we couldn't get data, we will be looking for extra data sources so we can add those. If we still cannot find anything about the book we may well pass them back to the requester to add them themselves.
If you are planning to index foreign language books yourself then (at least for now) you will have to index them in English as all our categories and ingredients are English. You can obviously type the recipe name in the original language. It is possible that at some time in the future we may offer foreign language versions of EYB but we have a hundred things we need to do before that.