A US 8 is not a EU 38 in every brand, and your shoppers have learned not to trust the label.
Cross-border apparel returns are higher than domestic ones, and the reason is not shipping. It is that a shopper in Berlin looking at a US size run has no reliable way to convert it, has been burned before, and orders two sizes to be safe. One of them is coming back, and you are paying for both legs.
Conversions are conventions, not conversions
The word conversion implies arithmetic. There is no arithmetic. A US 8 maps to a UK 12 and a EU 40 by convention, and individual brands drift half a size either side of that convention routinely. The standard mapping is a good starting point and a bad promise.
This is why the most useful column in any footwear chart is foot length. Regional shoe numbers are conventions; 26.7 centimeters is a measurement, and it means the same thing in every country you ship to.
Show one region, not all of them
The instinct is to publish every region in one wide table so nobody is left out. It is the wrong instinct. A shopper in Germany scanning five numeric columns to find theirs is doing work, and work at the point of decision is where carts get abandoned.
Showing the chart by country solves this: each market sees the columns that apply to it. The table gets narrower, the shopper finds their number faster, and nobody is converting anything in their head.
Units are the other half
A chart in inches for a shopper who thinks in centimeters is the same problem in a different costume. Automatic unit conversion switches the whole chart, and the quiz alongside it, so the numbers arrive in the units the shopper already reasons in.
Every mental calculation you leave for the shopper is a small opportunity to give up.
Publish the language too
If you are translating your product pages and leaving your size chart in English, the chart becomes the one part of the page a shopper cannot read at the exact moment they are least confident. Charts and quiz questions can be published in any of 140 languages, though you supply the translations rather than us machine-guessing them.
None of this is exotic. It is the same principle three times over: put the number in front of the shopper in the form they already understand, and stop making them do the conversion.