Do I understand it correcctly that this game has rules for BRP and Savage Worlds and has conversions for FATE and Trail of Cthulhu (Gumshoe)?
Yes. As mentioned above, the core books are dual statted. Some conversions to the other two systems exist.
How much of the book is dedicated to rules?
Quite a lot. Achtung modifies BRP pretty heavily by adding new skills and a specialized character generation process (I'm speaking of the CoC version, I'm not familiar with the Savage Worlds material, as I've only been a player in a short lived BRP A!C game).
What is the game like in general?
It's quite more stealth and combat oriented than CoC (at least the adventures we played which, IIRC, were from the official line). The basic plots were "Nazi mad scientist is creating a Lovecraftian superweapon, a British commando is sent to stop them".
Are there any supplements?
As mentioned above. The basic game allows you to play mostly British military men, other supplements should add combatants from other nations (the game is much more finicky about equipment and skills than the standard CoC, so professions are heavily specialized).
There is a campaign made of several linked adventures, as well as some larger scenarios.