public class MyTemplateBase : RazorEngineTemplateBase
{
    public IStringLocalizer Localizer;
}

string templateText = @"

    <h1>@Localizer[""Header""]</h1>

";
IRazorEngine razorEngine = new RazorEngine();
IRazorEngineCompiledTemplate<MyTemplateBase> compiledTemplate = razorEngine.Compile<MyTemplateBase>(templateText);
string result = compiledTemplate.Run(instance =>
{
    instance.Localizer = this.localizerInstance; // get instance from whatever source
});

public class MyTemplateBase : RazorEngineTemplateBase
{
    private IStringLocalizer localizer;
    public void Initialize(IStringLocalizer localizer)
    {
        this.localizer = localizer;
    }
    public string Localize(string key)
    {
        return this.localizer[key];
    }
}

string templateText = @"

    <h1>@Localize(""Header"")</h1>

";
IRazorEngine razorEngine = new RazorEngine();
IRazorEngineCompiledTemplate<MyTemplateBase> compiledTemplate = razorEngine.Compile<MyTemplateBase>(templateText);
string result = compiledTemplate.Run(instance =>
{
    instance.Initialize(this.localizerInstance); // get instance from whatever source
});

IRazorEngine razorEngine = new RazorEngine();
IRazorEngineCompiledTemplate compiledTemplate = razorEngine.Compile(templateText, builder =>
{
    builder.AddAssemblyReferenceByName("System.Security"); // by name
    builder.AddAssemblyReference(typeof(System.IO.File)); // by type
    builder.AddAssemblyReference(Assembly.Load("source")); // by reference
});
string result = compiledTemplate.Run(new { name = "Hello" });