And the language of the clients might not be the same. I might write in English to a German friend, but my whatsapp is localized to Danish and his to Germany.

How would whatsapp know what language to present the words in?

It could have a language picker. It's not like you're stuck with just one language; the underlying fingerprint could be converted into any language that has an appropriate dictionary constructed for it, so all you would have to do is ensure both parties have picked the same language and you can then compare the fingerprint easily.

That could get confusing.

"Ok, your pattern is Bread Cloud Tiller..." "Scheiße! We're being MITMed! My pattern is Brot Wolke Pinne..."

If I were to design this system, I'd make the language picker very prominent and have a very clear explanation saying to make sure both parties have the same language selected before comparing the words.

Alternatively, the party whose code is being confirmed would just select the language, and the other party's UI could automatically update to show the code in the chosen language. The first party would still have to pick a language both people understand but (as long as both parties have a working internet connection) there wouldn't be a sync issue.

I would just use numbers.

I might be misunderstanding the situation, but isn't emoji universal? Would it be ill-fitted here?

It could be a selectable option.

>> if you don't speak a common language with your chat partner then the app is useless anyway

> They do speak a common language, it's usually just not English.

I think you misunderstood the original poster was getting at. Here's the full parenthetical: "(perhaps with icons instead of specific words? if you don't speak a common language with your chat partner then the app is useless anyway)". So replace the words that you're supposed to read with little pictures. Then it doesn't matter what language the people speak as long as they're speaking the same language, since they'll be "reading" the images. You'll see a little house icon, and read "house" or "casa" or "jia" or whatever.