There are a few reasons why I haven't paid much attention to foreign stock markets up to this point. First is that the U.S. economy dwarfs all others, meaning that an economic event in the U.S. tends to have a larger impact on another given country than the other way around. In theory a crash in U.S. stocks could propagate to an otherwise healthy foreign market and show up there as an "out of the blue" anomaly. Thus, theoretically, as long as I can predict the U.S. market I'll more or less be predicting major moves in other markets as well.
A second closely related reason for focusing on U.S. markets and the S&P 500 index in particular is the sheer number of companies and total market capitalization of the NYSE and S&P index compared to other stock markets and indexes. The Nikkei average comprises 225 Japanese companies; the British FTSE index tracks 100 companies; the French CAC index tracks 40; and the German DAX index, like the Dow Jones Industrial Average, tracks only 30. Given that it is easier (in my experience) to predict a collective index than the stock price of an individual company, it follows that an index that averages 500 prices should be more "well behaved" than one that averages only 30.
Finally, I've also focused on U.S. stocks thanks to the greater availability of historical data. Presently I have prices for the Dow Jones Industrial Average dating back to 1896, along with more recent ancillary data that I use to help predict future market directions. By contrast the CAC 40 index (as an example) was first computed and published in 1987.
Today however, I'm realizing that by ignoring foreign markets I've missed an opportunity to show off my market timing skills. The incredible rise and fall of the Shanghai stock market over the past year - up 100% in 8 months and now down 27% in one month - would have been an ideal showcase of crash prediction during a period when the U.S. market has been positively boring. Now in hindsight I see the historical usefulness of the 1990 Japanese stock market crash that occurred in isolation without a commensurate crash in either the U.S. or in any other major market.
On my list of things to do I've added: "Track and forecast the major foreign stock markets!"