Pass the culture you have in the view back to the controller via the ajax call.
For instance:

...
$.ajax({...
    data:{..., culture: $("#currentCulture").val(), ...}
...});

In the controller, set Thread.CurrentThread.CurrentUICulture.

System.Threading.Thread.CurrentThread.CurrentUICulture = new CultureInfo(parameterInstance.culture);
var libararyItemInThatLanguage = library.GetDictionaryItem(yourKey);

 Maybe I misunderstand though, if you use library in the partial view, pass the culture to the view, set and reset CurrentUICulture there.

This is an interesting idea! I got my dictionary values correctly! But I'm still missing something, it might not actually be a problem but I want to get this straightened out hehe So here is what I have now.

My controller:

    public class FunController : Umbraco.Web.Mvc.SurfaceController
    {

        [System.Web.Mvc.HttpPost]
        public ActionResult Index()
        {
            return Content("ok");
        }

        [System.Web.Mvc.HttpPost]
        public ActionResult SuperFun(string text, string culture)
        {
            System.Threading.Thread.CurrentThread.CurrentUICulture = new CultureInfo(culture);
            return PartialView("UltraFunPartial");
        }
    }
}

Now my initial page has this mark-up:

<hr/><hr/><hr/>

<div id="culture" class="box">@Culture</div>

<div id="fun" style="min-height: 400px; background-color: springgreen;">
    @Html.Partial("UltraFunPartial")
</div>

<hr/><hr/><hr/>

Here @Culture, will display "fr-CA" on the initial page load. And the @Culture inside my partial view, will also display "fr-CA"

Here is my JS function now (I pick up the @Culture here basically):

function jsFun() {
    var culture = $("#culture").val();
    $.ajax({
        type: "POST",
        url: "/umbraco/surface/Fun/SuperFun/",
        cache: false,
        data: { culture: culture, text: "Some fun text" },
        dataType: "text",
        success: function (msg) {
            $("#fun").html(msg);
            // console.log("jsFun");
        },
        error: function (msg) {
            // console.log("Message: " + msg);
        }
    });
}

I ran in debug mode and my controller gets the culture string correctly, and sets the culture in CurrentUICulture correctly. But when my partial view renders, the @Culture is en-CA.

Here is my partial view:

@inherits Umbraco.Web.Mvc.UmbracoTemplatePage

@{
    // [...]
    var random = new Random();
    var randomNumber = random.Next(0, 100);
}

<div class="box" style="background-color: pink;">Culture in partial: @Culture</div>

<div>This should have text: (@library.GetDictionaryItem("General_EnSavoirPlus"))</div>
<div>And here is a random number: (@randomNumber)</div>
<div>CurrentPage: (@CurrentPage)</div>
<div>UmbracoContext.PageId: (@UmbracoContext.PageId)</div>

Now the fun thing is that my "General_EnSavoirPlus" dictionary label pops out correctly! This all feels a bit awkward though, what do you think, is this kosher?

Thanks for the help!

This is great! I'll keep this as reference, thanks a lot for the help!

pleasure :)

I just made a global filter attribute for this issue:

    public class AjaxRequestFilterAttribute : ActionFilterAttribute
{
    public override void OnActionExecuting(ActionExecutingContext filterContext)
    {
        var request = filterContext.RequestContext.HttpContext.Request;

        if (request.IsAjaxRequest())
        {
            var domain = ApplicationContext.Current.Services.DomainService.GetByName(request.Url.Host);

            if (domain == null) // trick to make it work on domains with port specified, like http://localhost:8080
            {
                domain = ApplicationContext.Current.Services.DomainService.GetByName(request.Url.Authority);
            }

            if(domain != null && !domain.LanguageIsoCode.IsNullOrWhiteSpace())
            {
                Thread.CurrentThread.CurrentCulture = Thread.CurrentThread.CurrentUICulture = new System.Globalization.CultureInfo(domain.LanguageIsoCode);
            }
        }
        base.OnActionExecuting(filterContext);
    }
}

This way I don't have to worry on each ajax request about culture and labels. Also this code will run only if it's an ajax request. Seems to work fine so far. I'm a bit worried it would interfere with umbraco's internal controllers (if any), but didn't notice a problem yet.

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