I have owned many freezers and fridges in my life, an it was always "higher number = colder". Which is usually completely obvious, because the dial includes a 0 position, which shuts it off.
For example, if you notice on older cars there is a dial for the air conditioning. On that dial there is a gradually increasing line of thickness (in blue in my mother's 2010 Camry). Increasing makes the car colder, or rather it makes the compressor deliver more cold air into the vehicle.
So .. why is this dial on this freezer like that? Because I think at one time in history we viewed air conditioning as a manufactured force. Increasing the number is to increase the intensity of that mechanical force.
Arrow/tick mark on the panel.
The bonus is this works in all temperature scales and language systems and probably includes most perception disabilities---blind, dyslexic, illiterate, etc. But I could still imagine a handful of people messing this one up.
(Also, if the thing has an off mode... how do you mark and emboss that? A giant X? Well that looks like a snowflake, especially to your fingertip! A zero? A small horizontal line? Or, probably best, just a separate on/off switch...)
If you live in a snow-prone area you'll know that large snowflakes (which are actually clumps of flakes which happened to meet and merge on the way down) mean higher temperatures while small flakes equate to lower temperatures. So... Put it on the big flake to raise the temperature?
Make it a separate control entirely.
I had been unsure about this for a number of years, until one day I decided to check by slowly turning the knob to both extremes and listening for when the compressor kicked on and shut off.