Validation messages from custom model validation attributes are locked to first loaded language - asp.net-mvc-4

Question

I am working on a multi lingual website using Umbraco 7.2.4 (.NET MVC 4.5). I have pages for each language nested under home nodes with their own culture:
Home (language selection)
nl-BE
some page
some other page
my form page
fr-BE
some page
some other page
my form page
The form model is decorated with validation attributes that I needed to translate for each language. I found a Github project, Umbraco Validation Attributes that extends decoration attributes to retrieve validation messages from Umbraco dictionary items. It works fine for page content but not validation messages.
The issue
land on nl-BE/form
field labels are shown in dutch (nl-BE)
submit invalid form
validation messages are shown in dutch (nl-BE culture)
browse to fr-BE/form
field labels are shown in french (fr-BE)
submit invalid form
Expected behavior is: validation messages are shown in french (fr-BE culture)
Actual behavior is: messages are still shown in dutch (data-val-required attribute is in dutch in the source of the page)
Investigation to date
This is not a browser cache issue, it is reproducible across separate browsers, even separate computers: whoever is generating the form for the first time will lock the validation message culture. The only way to change the language of the validation messages is to recycle the Application Pool.
I doubt that the Umbraco Validation helper class is the issue here but I'm out of ideas, so any insight is appreciated.
Source code
Model
public class MyFormViewModel : RenderModel
{
public class PersonalDetails
{
[UmbracoDisplayName("FORMS_FIRST_NAME")]
[UmbracoRequired("FORMS_FIELD_REQUIRED_ERROR")]
public String FirstName { get; set; }
}
}
View
#inherits Umbraco.Web.Mvc.UmbracoTemplatePage
var model = new MyFormViewModel();
using (Html.BeginUmbracoForm<MyFormController>("SubmitMyForm", null, new {id = "my-form"}))
{
<h3>#LanguageHelper.GetDictionaryItem("FORMS_HEADER_PERSONAL_DETAILS")</h3>
<div class="field-wrapper">
#Html.LabelFor(m => model.PersonalDetails.FirstName)
<div class="input-wrapper">
#Html.TextBoxFor(m => model.PersonalDetails.FirstName)
#Html.ValidationMessageFor(m => model.PersonalDetails.FirstName)
</div>
</div>
note: I have used the native MVC Html.BeginForm method as well, same results.
Controller
public ActionResult SubmitFranchiseApplication(FranchiseFormViewModel viewModel)
{
if (!ModelState.IsValid)
{
TempData["Message"] = LanguageHelper.GetDictionaryItem("FORMS_VALIDATION_FAILED_MESSAGE");
foreach (ModelState modelState in ViewData.ModelState.Values)
{
foreach (ModelError error in modelState.Errors)
{
TempData["Message"] += "<br/>" + error.ErrorMessage;
}
}
return RedirectToCurrentUmbracoPage();
}
}
LanguageHelper
public class LanguageHelper
{
public static string CurrentCulture
{
get
{
return UmbracoContext.Current.PublishedContentRequest.Culture.ToString();
// I also tried using the thread culture
return System.Threading.Thread.CurrentThread.CurrentCulture.ToString();
}
}
public static string GetDictionaryItem(string key)
{
var value = library.GetDictionaryItem(key);
return string.IsNullOrEmpty(value) ? key : value;
}
}

Answer 1

So I finally found a workaround. In attempt to reduce my app to its simplest form and debug it, I ended up recreating the "UmbracoRequired" decoration attribute. The issue appeared when ErrorMessage was set in the Constructor rather than in the GetValidationRules method. It seems that MVC is caching the result of the constructor rather than invoking it again every time the form is loaded. Adding a dynamic property to the UmbracoRequired class for ErrorMessage also works.
Here's how my custom class looks like in the end.
[AttributeUsage(AttributeTargets.Property | AttributeTargets.Field | AttributeTargets.Parameter,
AllowMultiple = false)]
internal class LocalisedRequiredAttribute : RequiredAttribute, IClientValidatable
{
private string _dictionaryKey;
public LocalisedRequiredAttribute(string dictionaryKey)
{
_dictionaryKey = dictionaryKey;
}
public IEnumerable<ModelClientValidationRule> GetClientValidationRules(
ModelMetadata metadata, ControllerContext context)
{
ErrorMessage = LanguageHelper.GetDictionaryItem(_dictionaryKey); // this needs to be set here in order to refresh the translation every time
yield return new ModelClientValidationRule
{
ErrorMessage = this.ErrorMessage, // if you invoke the LanguageHelper here, the result gets cached and you're locked to the current language
ValidationType = "required"
};
}
}