added culture fallback (#5041)

aspnetcore/fundamentals/localization.md

@@ -121,7 +121,7 @@ In the preceeding code, `SharedResource` is the class corresponding to the resx
 ASP.NET Core allows you to specify two culture values, `SupportedCultures` and `SupportedUICultures`. The [CultureInfo](https://docs.microsoft.com/dotnet/api/system.globalization.cultureinfo) object for `SupportedCultures` determines the results of culture-dependent functions, such as date, time, number, and currency formatting. `SupportedCultures` also determines the sorting order of text, casing conventions, and string comparisons. See [CultureInfo.CurrentCulture](https://docs.microsoft.com/dotnet/api/system.stringcomparer.currentculture#System_StringComparer_CurrentCulture) for more info on how the server gets the Culture. The `SupportedUICultures` determines which translates strings (from *.resx* files) are looked up by the [ResourceManager](https://docs.microsoft.com/dotnet/api/system.resources.resourcemanager). The `ResourceManager` simply looks up culture-specific strings that is determined by `CurrentUICulture`. Every thread in .NET has
 `CurrentCulture` and `CurrentUICulture` objects. ASP.NET Core inspects these values when rendering culture-dependent functions. For example, if the current thread's culture is set to "en-US" (English, United States), `DateTime.Now.ToLongDateString()` displays "Thursday, February 18, 2016", but if `CurrentCulture` is set to "es-ES" (Spanish, Spain) the output will be "jueves, 18 de febrero de 2016".
 
-## Working with resource files
+## Resource files
 
 A resource file is a useful mechanism for separating localizable strings from code. Translated strings for the non-default language are isolated *.resx* resource files. For example, you might want to create Spanish resource file named *Welcome.es.resx* containing translated strings. "es" is the language code for Spanish. To create this resource file in Visual Studio:
 
@@ -169,19 +169,21 @@ Resource files using `@inject IViewLocalizer` in Razor views follow a similar pa
 
 If you don't use the `ResourcesPath` option, the *.resx* file for a view would be located in the same folder as the view.
 
-If you remove the ".fr" culture designator AND you have the culture set to French (via cookie or other mechanism), the default resource file is read and strings are localized. The Resource manager designates a default or fallback resource, when nothing meets your requested culture you're served the *.resx file without a culture designator. If you want to just return the key when missing a resource for the requested culture you must not have a default resource file.
+## Culture fallback behavior
 
-### Generating resource files with Visual Studio
+As an example, if you remove the ".fr" culture designator and you have the culture set to French, the default resource file is read and strings are localized. The Resource manager designates a default or fallback resource for when nothing meets your requested culture. If you want to just return the key when missing a resource for the requested culture you must not have a default resource file.
+
+### Generate resource files with Visual Studio
 
 If you create a resource file in Visual Studio without a culture in the file name (for example, *Welcome.resx*), Visual Studio will create a C# class with a property for each string. That's usually not what you want with ASP.NET Core; you typically don't have a default *.resx* resource file (A *.resx* file without the culture name). We suggest you create the *.resx* file with a culture name (for example *Welcome.fr.resx*). When you create a *.resx* file with a culture name, Visual Studio will not generate the class file. We anticipate that many developers will **not** create a default language resource file.
 
-### Adding Other Cultures
+### Add Other Cultures
 
 Each language and culture combination (other than the default language) requires a unique resource file. You create resource files for different cultures and locales by creating new resource files in which the ISO language codes are part of the file name (for example, **en-us**, **fr-ca**, and **en-gb**). These ISO codes are placed between the file name and the *.resx* file name extension, as in *Welcome.es-MX.resx* (Spanish/Mexico). To specify a culturally neutral language, remove the country code (`MX` in the preceding example). The culturally neutral Spanish resource file name is *Welcome.es.resx*.
 
 ## Implement a strategy to select the language/culture for each request  
 
-### Configuring localization
+### Configure localization
 
 Localization is configured in the `ConfigureServices` method:
 
@@ -233,7 +235,7 @@ If you only specify one of culture info and UI culture, the specified culture wi
 
 The [Accept-Language header](https://www.w3.org/International/questions/qa-accept-lang-locales) is settable in most browsers and was originally intended to specify the user's language. This setting indicates what the browser has been set to send or has inherited from the underlying operating system. The Accept-Language HTTP header from a browser request is not an infallible way to detect the user's preferred language (see [Setting language preferences in a browser](https://www.w3.org/International/questions/qa-lang-priorities.en.php)). A production app should include a way for a user to customize their choice of culture.
 
-### Setting the Accept-Language HTTP header in IE
+### Set the Accept-Language HTTP header in IE
 
 1. From the gear icon, tap **Internet Options**.
 
@@ -249,7 +251,7 @@ The [Accept-Language header](https://www.w3.org/International/questions/qa-accep
 
 6. Tap the language, then tap **Move Up**.
 
-### Using a custom provider
+### Use a custom provider
 
 Suppose you want to let your customers store their language and culture in your databases. You could write a provider to look up these values for the user. The following code shows how to add a custom provider:
 
@@ -278,7 +280,7 @@ services.Configure<RequestLocalizationOptions>(options =>
 
 Use `RequestLocalizationOptions` to add or remove localization providers.
 
-### Setting the culture programmatically
+### Set the culture programmatically
 
 This sample **Localization.StarterWeb** project on [GitHub](https://github.com/aspnet/entropy) contains UI to set the `Culture`. The *Views/Shared/_SelectLanguagePartial.cshtml* file allows you to select the culture from the list of supported cultures: