I'm a native English-speaker (US). I'd say it's very common to see
"chainmail" here, and though I don't claim to be an expert it
would seem to me that the word originated with the double-l and the
e, so why not spell it that way. Therefore "chainmaille" is the
version I choose to use. I will say, though, that if someone said
they were making "mail" I'd wonder what the heck they were talking
about, that is something that is sent through the postal service.
If someone, however, says they are making "maille", I don't believe
there's any confusion there.
There’s English and there’s English.
E.g. Aluminium and Aluminum
The word didn’t actually originate with a “lle”, but if people want
to choose a term that is recognised as a pseudo brand name who am I
to argue. I’m an English speaker and I choose to use the English
"Mail, mayle or chain was the English name for it, maille was the
French version, and it was called malien in Flemish and Dutch. This
and the alternative spellings maile and maille derive through the
Italian maglia, from the Latin macula, meaning “mesh of a net”. The
Spanish corresponding word is malla and in Portuguese it is malha.
The Welsh term lluric derives from the Latin lorica.
Many modern American armourers prefer the French spelling “maille"
in order to avoid confusion with the term chain letter for
’chainmail’ or postal delivery for ‘mail’.”
Regards Charles A.