How to re-structure a multi-language website to keep rank?

Let me ask you (mainly the SEO and Adsense experts) this and if you think you could do it better/would do it different:

I have feldzug.net, a website regarding World of Tanks gamers and providing useful statistics for their gameplay.

The page is visible in German and English, however there is no dinstict URL for every language at this moment. Both languages are served by the same URL.

Regardless, Google has managed to index both versions (at least for the front page as far I'm aware so far) and they both rank for several keywords at TOP 1-7 in the german and english search. The current audience is about 75% german and 25% from other countries, with USA & UK being at about 10-15%.

So here is what I want to achieve:

• Increase visibility on english speaking audiences,
• being able to target Adsense/other ads or campaigns to specific languages
• while maintain the rankings still, regardless of any structural changes
• being able to organize different contents per language
• and maybe add more languages based on the other countries I have visitors from

So my first thought was, to change the whole site to distinct URLs for every language. Now there are several ways to accomplish this from the technical side and I'm skilled in all of these.

What I'm worried about is I might influence my rankings negatively by doing this.

What would happen if I redirect the front page (which is the one ranking so well) to different URLs for each visitor language, in regard of SEO? Say, if there would be no World of Tanks Stats (WoT WN8, FXR, WN7, Efficiency, Rating, Stats) anymore but only http://www.feldzug.net/en/ and http://www.feldzug.net/de/ (it would redirect to one of these).

Could this affect the rankings in Google in some unwanted way?
How would Google react?
Would this make a change in regard of Adsense, to see pages more relevant to specific contents and their language?

How would you go about this?