I don't think I'd point to the alliance between Germany and Japan as an example of a strong alliance despite language differences. These two nations were allied for no other reason that the old adage "the enemy of my enemy is my friend". Both regimes despised communism and were held in extreme suspicion by the Wester Powers. Their alliance was one of convenience and little more. The Germans, with their racial ideas, could hardly come to fully embrace an Asian empire as their equal and full partner. There was very little interaction between the two nations and no real coordination in terms of strategy or tactics.

As to what would have happened if the Western Allies had declared war on the Soviet Union in 1939, well, I think the governments of both France and Britain would have likely fallen. Taking on Germany was tough enough; with Russia added on the task would have seemed overwhelming and that would call into question the leadership. It would have been a very, very unpopular war. Britain would, in this scenario, have to significantly increase the ground forces it held in the Middle East and India if it did not want to lose these vital territories, which would now be under threat from Soviet forces to the north. As Kai-Petri has pointed out, a major theater of war would have likely been the Scandinavian countries and simple geography here would tend to favor the Germans and Russians, given how much closer they were to these areas. And if Britain already had to send off a lot of its reserves to reinforce the Middle East and India, how would they come up with a force to seriously threaten both Germany and the Soviet Union in Scandinavia? As far as I see it, this "What If?" scenario definitely favors the Germans.

How long would this alliance between Nazi Germany and the Soviet Union would last? I can't say but I imagine it would have lasted, as in the real world, just as long as the two countries faced immediate danger. Once France was out of the picture (and there is little doubt it would fall in this scenario) Germany would still be left unable to cross the Channel and so Germany would start looking east again. How long Hitler would be willing to postpone Barbarossa is unclear, but I imagine he would be less quick to attack Russia if it was aiding his cause by keeping the Brits occupied out east. However, he may well have taken the opportunity to stab Stalin in the back while the Red Army had fewer units guarding the western borders.

Would Stalin have even trusted Germany enough to attack Britain in India and the Middle East? Frankly, I doubt Britain could have mustered enough force to be a major threat to Siberia (and what would be the point of advancing into that huge, relatively unpopulated wilderness?) so I imagine the situation between Siberia and the Middle East and India would have been a sitzkrieg. I doubt there would be much action but in case there were both sides would have to station considerable forces in the threatened areas to act as a deterent. Hitler would still have to attack the Soviet Union at some point (because, being Hitler, that was a raison d'etre of his life) and when he did it would be interesting to see just how quickly Stalin would try and patch things up with the Allies. And, I doubt that the Allies would mind dropping the war against Russia in return for an ally against Germany. And so we're back to basically the historical scenario. All in my humble opinion, of course.