But yea Panzer und girls is sort of realistic. There is a big focus on German tanks though. It makes me sad to not see enough British tanks (although I do love German tanks as well).
They also have a manga only battle with Italian tanks as well. I'm pretty happy with Italian tanks getting the love they deserve. The Carro Armato P 40 is certainly a nice tank. Sad that they didn't give much to the Italian Tank destroyers. The Semovente 105/25 is quite cool.
Meh, most of it comes down to the writer's choice, others (I suspect) are due to whether the tanks in question might be competitive enough in the field.
For example: the British and Italian designs. For the latter, Italy mainly deployed a combination of tankettes
(think of a lightly armoured assault gun) and light and medium armour during the war. Charitably speaking, they worked great against infantry-only units, but the moment anything with so much as a 2 pound anti-tank gun showed up, 75% of the time, they went up in smoke. Some Italian designs, however, did cause a few headaches for the Allies who fought them later on, but compared to the German designs, they were comparatively inferior.
The British, admittedly, fell behind the other powers (despite being one of the pioneering nations of armoured warfare) at the outbreak of conflict, so a lot of their early tanks were focused around either infantry support (like the Matilda II), light reconnisence (such as a Vickers' design, whose name escaped me) or were of the 'cruiser' doctrine (A15 Crusader, Cruiser I and even influenced the Cromwell) - you may not be able to outfight your opponent pound for pound, but you can run rings about him.
Later British designs, such as the Churchill, Cromwell and Sherman VC, overcame a number of gaps and/or were designed for specific tasks in mind.