Associate css file at runtime through your code. Based on the need/language selected, add the CSS file.
This means, you need to provide the source of CSS file at runtime.
HtmlLink css = new HtmlLink();
css.Href = "css/myCurrentLanuageStylesheet.css";
css.Attributes["rel"] = "stylesheet";
css.Attributes["type"] = "text/css";
css.Attributes["media"] = "all";
Page.Header.Controls.Add(css);
You can also add it using Javascript, details here:
Adding Javascript and CSS files at run time[
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