If you happen to be using ASP.NET MVC 4 onwards, check this post:
Localizing Default Error Messages in ASP.NET MVC and WebForms
Basically you have to add the following piece of code in your Application_Start() method in Global.asax:
ClientDataTypeModelValidatorProvider.ResourceClassKey = "Messages";
DefaultModelBinder.ResourceClassKey = "Messages";
Right click your ASP.NET MVC project in Solution Explorer inside Visual Studio and select Add => Add ASP.NET Folder => App_GlobalResources.
Now add a .resx file inside this folder called Messages.resx.
Finally add the following string resources in that .resx file:
Name Value
==== =====
FieldMustBeDate The field {0} must be a date.
FieldMustBeNumeric The field {0} must be a number.
PropertyValueInvalid The value '{0}' is not valid for {1}.
PropertyValueRequired A value is required.
You should be good to go.
Note that the value you’re interested in is the FieldMustBeNumeric. To localize it to Spanish, you have to add another resource file named Messages.es.resx. In this specific .resx file replace the resource value with:
Name Value
==== =====
FieldMustBeNumeric El campo {0} tiene que ser numerico.
If you happen to be using ASP.NET MVC 3 downwards, this solution can help you achieve the same result: https://stackoverflow.com/a/2551481/114029