There's talk of wizards abroad and we see the students from Beauxbatons and Durmstrang, but if Voldemort really is trying to take over the world of magic, why was there never any sign of any wizard armies from abroad coming over to help?
Fear. Politics. Fear. Closer to Home. The Chosen One. Psychology.
No other country wanted to try and help through fear if Voldemort won then who would be next? The country that helped the people standing in his way.
You have to remember that the British Ministry of Magic fell. That would instill fear into the Ministry of Magic equivalents in other countries. Also the wizarding worlds don't really have armies, the Ministry has Aurors, up until The Battle Of Hogwarts only the Order of The Phoenix stood in Voldemort's way and only a few members were known openly. (It can be argued all good teachers were also members of the OOTP however not openly)
Yet more fear, Dumbledore was known to be the most powerful wizard of the time although I would say the general consensus was torn between him and Voldemort. When Dumbledore died I would imagine this would strike fear into the hearts of anyone who had ever met him.
The question about international wizards arises a lot but when you think about it how many English wizards actually helped out in the fight against Voldemort? Not many at all, people hid from him and from his pure blood regime but most did not fight. Only the Aurors and the Order fought.
England also had what the media had dubbed "The Chosen One" he would no doubt save the day. The hope of the free world was resting upon Harry's shoulders and if he couldn't do it who could?
Also the Bystander Effect: it is a lot easier to stand by and do nothing if everyone else around you is standing by and doing nothing. More keywords include Diffusion of Responsibility the more people are witnessing the less the responsibility you feel to actually do something. So for all the countries standing by they could all say well "no one else is helping out why should we". Interesting to read up on would recommend.
No doubt Voldemort wanted to eventually secure world domination, but he would not have been so stupid as to advertise this fact. If anyone asked, he would have said he was simply trying to become the wizardly ruler of Britain.
Wizards in other countries might have:
The other side of this coin is that the British MM would have been reluctant to allow interference in what it saw as internal British affairs. Like politicians anywhere, they are proud and protective of their independence. A team of international Aurors marching in to deal with Voldemort would not necessarily have been welcome.
For a historical parallel, consider the rise of the Nazi party in Germany. Some people outside Germany were concerned; but others were ambivalent or positively welcomed Hitler's seizure of power. Even those who opposed Hitler never seriously considered a military invasion of Germany to prevent him from becoming ruler in 1933. They only went to war in 1939, when they were no longer willing to tolerate Hitler's external aggression.