I'd ask the customers you do have why they downloaded the app, what they were looking for when they found it, how they heard about it, what they like/don't like about it, and how they think you could reach more people. What pain are you solving, who experiences it the most, and how do you reach those people?? Answer those questions and you're on your way.
You need to setup a Pay Per Click (PPC) campaign, targeting areas like Germany.
You'll need to build a good keywords list and have integration with Analytics to maximize the efficiency of your campaign.
If you have any detailed questions feel free to contact me.
If you can't get traction, one reason is your app is not getting exposure, is not attracting Germans who would be interested.
I have struggled in the German market as well. Germans are a bit less excitable and a bit more skeptical.
I would suggest thinking about the following:
1) the app name can be English - no problem - but advertizing and descriptions need to be in good German.
2) Find someone wiht fluent German who is active in social media. Create a Facebook group and page for German speaking FB users. Provide, if possible, a demo.
3) German language YouTube video!
4) A network comparable to LinkedIn but dominated by German users is XING. You can join approriate groups there.
5) Of course, if you're willing to spend some money, FB ads are good, but you want to know how to define the right target groups.
One thing that could help with all of this is to find a German speaking app enthusiast who can help you promote using all the suggestions above.