Nah. "Walchensee" is a German word, so when you use it you are indeed communicating in German (even though you appear to not realize it). Adding "lake" is just translating part of the word. But if you're gonna translate part of it, then you need to take the translated part off of the German word. So sure, add "lake," then it would be "Walchen Lake" or "Lake Walchen" (much like how the wikipedia article for that body of water does it). Otherwise you say Lake Lake Walchen, which makes no sense. Either translate or don't... but ya gotta pick one, not go halfway.

As an illustrative analogy... if you were talking about Lake Geneva and decided to use the French name, would you say "Lake lac Léman" because English speakers might not know that "lac" means "lake"? I don't think you would.