You should be able to iMessage while connected to WiFi in Germany. If you are trying to send a message using iMessage and it is attempting to send it as a text message, you may need to check that the iMessage function on your phone is enabled. Below I have provided steps on how to enable iMessage on your iPhone.
1. From the home screen, select Settings
2. Select Messages.
3. Drag the iMessage switch left or right to enable / disable iMessage.
If iMessage is enabled and you are still unable to use it, please post back and I will be more than happy to further assist you.
Also be sure that the person you are trying to iMessage is using an iDevice (iPhone, iPod touch or iPad), iMessage doesn't support non-Apple devices (eg. Android or Windows Phone devices), if you try to iMessage a non-Apple device, it will attempt to text the number instead
If it comes out blue then you in the clear. If it comes out green then you will be charged international text price.
If you do bulk texting(more than 1 person) and one of them does not use an iDevice then it will come out of your texting package(if you have one) or charged PPU(pay per use).
You can turn off sending it through SMS in the messages settings.