If you are using the SPUrl expression prefix you can use the ~language to get the language-code for the site.
Example:
<link id="LanguageCss" rel="stylesheet" type="text/css" href="<% $SPUrl:~SiteCollection/Style Library/TheCompany/~language/LanguageSpecificStyle.css%>" runat="server" />
Will give you something like this when evaluated:
<link id="ctl00_LanguageCss" rel="stylesheet" type="text/css" href="/sites/theSite/Style Library/TheCompany/en-US/Styles/LanguageSpecificStyle-main.css" />