Thanks, I'm waiting for the official answer of the Embassy now.
If you study in England i don't think you will have a problem concerning English.
But still you should be careful. Usually people mistake in simple things.
About the other tests i can't help you to much as a graduate it was not necessary for me to pass them. I had to do a research project.
You have one hour for each test so you should not lose time with questions or problems you don't know the answer. Let them for the last minute and if you can't figure out an answer just guess. From what I saw there is math, physiques and biology of high school. So nothing for olimpics or staff like that. You could use you high school course book to prepare. Maybe your Croatian books will be better. From what I could see in the Balkans, south-east Europe, Russia we start to do some things earlier so the course books might contain more information. Well this is a personal observation you should know better.
In my case i needed to personally contact an university and ask for a letter of acceptance but you won't need to do that. undergraduate who don't speak Japanese do a year of Japanese in Japan and then are place in an university according to their choices but also according to the results of Japanese tests. I hear that some university may ask you to give some other entrance exams too. Where you do your Japanese courses it seams is up to them. At list the undergraduate Romanians couldn't chose the city or stuff like that. I'm assuming is similar everywhere.
The main thing in the interview is seams to be your reasons for wanting to study in japan. I guess they don't want to chose someone that later might change his mind. Well they ask me about my research program also but it won't be your case. I get also a question like what club I will want to join there. It seams in japan being in a club is important for your social life, integration and so on. Do a research about their educational system, way of life and universities. It's good to know a number of universities that have your field of study. Be ready to explain why you want to study there and what you want to do in the future.
After graduation i thing is up to you what you will do. It seams you can continue your studies with a master there if the teachers accept to give you a recommendation for receiving Monbusho for master also.
I don't think your field of study will make you chances smaller if you have very good results to the test and interview. I know graduate Romanians that got there to study some very particular things in physique that you wont find in every university so i assume it's not a selection criteria.
One thing you must also consider . As a undergraduate is much more difficult to get the scholarship because the number of scholarships is somewhere between 400 and something for everybody. For graduate are about 4000 and something. From what I saw the country you come for might be helpful. It seams they prefer people from what they consider developing countries. Also consider that many of this scholarships go to Asian countries.
Be prepare to wait a lot till you have an answer. I've waited 7 month now and still I'm waiting.