If you have totally different stylesheets you can load them by the current selected language by setting the visibility on the HTML Head webpart. Something like:
LocalizationContext.CurrentCulture.CultureCode == "en-GB"
Or you could name your language specific stylesheets with a similar fashion like Site1Stylesheet-en-GB
then in your HTML Head webpart just have the URL with the dynamic macro at the end.
<link href="/CMSPages/GetResource.ashx?stylesheet=Site1Stylesheet-{%LocalizationContext.CurrentCulture.CultureCode|(identity)GlobalAdministrator%}" type="text/css" rel="stylesheet" />