For this achievement, you need to have 5 German tanks in your garage at the same time. All type and Tier of tanks count. Please note that you can't buy twice the same tank, so they have to be different.
The easiest and earliest way to get this is to take the LTraktor and research the 2 packs called "VK 31 (K)" and "Leichttraktor II", and then all the tanks "G. Pz MK. VI", "PzJg I", "Pz 35 (t)", and the "Pz II)". You will need LTraktor experience to do the research, and you earn it by playing matches with this tank. To research modules and tank, select the LTraktor in the garage and press (X), then select the first option.
Once those tanks are researched, you can buy the 4 of them in the garage for silver. You earn silver by playing matches with any tank.
If you don't have enough slot in your garage, I suggest you to sell any other Tier I tank because they don't cost anything to buy back if you need them later. Please note that there is a limit of tanks you can sell every 24H (4 or 5 I think), so don't go for all of nation related achievements at once.
The achievement pops right after in the garage.
PS: as I have already researched those tanks, I don't know the exact amount of XP and silver to buy the tanks, please leave a comment if you do.