why do sea chanties seem to be mostly used for folk music of the english speaking countries?

Answer 1

I was tempted to simply say that there are two obvious reasons for the apparent preeminence of English as the language for the shanty / chantey: -
[1] That the sea occupied such an enormously important place in the history of many English-speaking countries - especially during the age of sail, which was of course when the shanty as a work-song had its origin.
[2] That English as a language is so dominant in today's world, that it has tended to blot out the memory of non-English shanties.

But, on doing a spot of research, I found that I was wrong about that ... or at least not completely correct. To my surprise, I found that shanties are actually more popular (at least today) in some non-English cultures than they are amongst English speakers.

Although shanties form part of the repertoire of many folk-singers throughout the English-speaking world, in some other European countries there are actually Shanty Choirs who are dedicated to performing shanties in their own languages. These specialist choirs are popular in countries such as Poland, Germany, the Netherlands and Norway.

For example, one Polish shanty-band, EKT-Gdynia has been performing for the past 20 years and has made at least 4 record albums. Then there is Kat'n t'Seil, a Dutch group dedicated to sea songs. Both Poland and the Netherlands also host festivals dedicated to sea shanties, such as "De Slag op 't Wiede" and "Szanty we Wroclawiu".

So ... thanks for asking a very good question ... from which I learned something that I would otherwise never have suspected.

Answer 2

Or are folk tunes used for sea shanties?

There are many versions of most folk songs. Australian bush ballads tend to use English and Irish folk tunes.

Doesn't really answer your question, but engrafting different words onto existing tunes is common for folk songs.