I use multilingual and boost module in drupal system,
but i didn't use prefix for my language, use new domain for different language such as below:
how it let language domain works with boost module, i use 6.x-1.18 version for boost module.
i check the cache, normal folder, all static file for different domain language will get same HTML files. can't have any nl.test.com or no.test.com folder be created.