Which regions of the world prefer mustard in their mayonnaise?
The difference in Mayonnaise is varied. For example, in the USA Hellman's and Best Foods Mayonnaise (Same company by the way and same product) add sugar to reduce the acidity. Regional tastes are also taken into account by the manufacturers.

Hellman's mayo in Europe has different ingredient percentages than the same mayo uses for the American market. Many in North America find Duke's mayo to be superior in taste and use than Hellman's due to the ingredient mix. Hellman's mayonnaise in the USA does NOT contain mustard, but there is nothing stopping you from adding a dollop of Dijon in the mix if that's your desired taste.

As a side note, in Japanese cooking where mayonnaise is called for, the most common brand is Kewpie (available in Asian stores and Amazon). The primary difference is that Kewpie mayo only uses egg yolks and also rice vinegar instead of distilled vinegar. It can be used for any recipe that calls for mayo and has a really delightfully more bright taste that's a little different than other mayo's. 

Kraft now also offers Avocado Oil Mayonnaise. Looks healthy until you read the whole list of ingredients. Ugh!

Whatever you choose, you can certainly modify to taste. Bon Appetit!