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!

From experience, mayo across Europe differs widely from country to country. Even within a country, the differences brand to brand are huge. Just compare the colours (hey, that tells you I'm in the UK) of various brands. In the UK Hellmans is not mustarded (Rapeseed Oil (78%), Water, Pasteurised Free Range Egg & Egg Yolk (7.9%), Spirit Vinegar, Salt, Sugar, Lemon Juice Concentrate, Antioxidant (Calcium Disodium EDTA), Flavourings, Paprika Extract), Heinz 'Real Mayonnaise' (Rapeseed Oil 68%, Water, Pasteurised Egg Yolk* 5%, Spirit Vinegar, Sugar, Starch, Salt, Mustard Seeds, Spices, Antioxidant (Calcium Disodium EDTA), *From Free Range Eggs.) has mustard, slightly yellower in colour and (imho) tastes better. Same for other brands... they vary. However none of the mass-market mayos on sale in the UK are as flavoursome as French mass-market mayo. Take for instance Benedicta (Vegetable oil - water - egg yolks, fresh (5%) - Dijon mustard - vinegar - salt - sugar - modified corn starch - thickener: xanthan gum - color: beta carotene - aroma.)

And please don't look at the ingredients too closely in those brands... they're all extended with various things that have no place in real mayo!

So far generic-American mayonnaise, polish mayonnaise, Japanese mayonnaise, and French (I presume) mayonnaise, are all I've come across with the polish one being the only one I remember having mustard as an ingredient.

The difference I noticed with "French" mayo was that it was made with lemon juice rather than just vinegar. (I vastly prefer this style myself, but can't get it regularly)

If you actually want to get your hands on some polish majonez in Canada, I'd suggest looking for a Polish/Baltic food shop nearby. (or "ethnic section" of a supermarket)

I can only speak for Europe, but I've seen majonez on sale most of the times I've headed into one. It likely won't be super cheap, but should suffice to get your fix.

Original recipe have just olive oil, egg yolks and a pinch of vinegar or lemon juice.

It originated in the city of Mahon in the balearic isle of Menorca and brought to France after the invasion of the isle by french troops in 1756.

In Spain mayonnaise (or mahonesa) doesn't contain any mustard.

In italy mustard is a thing, mayonnaise is another thing they are really different. Basically mayo in italy is eggs and oil (you can add salt, vinegar etc... but they are not necessary... also it can be olive oli, seed oil etc... and you can decide to only use yolks). Mustard is not an italian thing, you will find it anyway but not under mayo.

PS. I like to mix italian mayonnaise with ketchup (or rubra, an italian kind of ketchup) to create "salsa rosa" (pink sauche). It taste great and it's my favourite with fried and not fried potatoes.