I needed a domain based language switch on a site last night so thought I would post the solution here for future use.

## English Setting First

[globalString = IENV:HTTP_HOST = *url.co.uk]
config {
    sys_language_uid =  1
    language = en
    locale_all = en_EN
    htmlTag_langKey = en
    baseURL = www.url.co.uk
}

## Dutch Setting
[globalString = IENV:HTTP_HOST = *url.nl]
config {
    sys_language_uid =  0
    language = de
    locale_all = de_DE
    htmlTag_langKey = de
    baseURL = www.url.nl
}
[end]