Meaning of "No Russian" in Call of Duty: Modern Warfare 2

What was meant by "Remember - No Russian" in Call of Duty: Modern Warfare 2?

Did it mean "Don't Speak in Russian"? Or "Don't leave any Russians alive"?

A Wikipedia article mentioned that a Japanese translation of the game translated it as something like kill them all, they are Russian, and that the game's producer said it was a mistranslation but they wouldn't fix it.

Did any translations of the game into other languages translate it to have the other possible meaning (no Russian language)?

Also, "No Russian" in the sense of "Don't leave any Russians alive" would be mildly ungrammatical. Was the ultranationalist who said this speaking in English, and if so, was he bad at English?

@SilverStreak answered your main question adequately already, but to answer your question about whether the ultra-nationalist (It's been a while, but I believe he was named Makarov) is bad at English, the answer is no. Makarov was portrayed as a highly skilled individual, possibly even a genius in a way, if it wasn't for his twisted morality. It is very unlikely that during such an extremely important false-flag operation that he would skimp on learning English -- that would be half-assing it. He also speaks flawless English for the rest of the game.

As they were trying to frame the US as being behind (or involved in) the incident, I think that the phrase 'No Russian' can be interpreted as meaning 'Do not speak Russian'. It might be translated differently in other languages but as English is the original language, I think that we can safely disregard the translations that would go against this.

This is at least how I took it when playing the game for the first time and never considered that it could be taken any other way.