Auto-detect language on payment page (#2552)

* Auto-detect language on payment page

based on the requst Accept-Language header, which is the language you configured in your browser/OS and this 99.99% accurate

* Update BTCPayServer/Services/LanguageService.cs

Co-authored-by: britttttk <39231115+britttttk@users.noreply.github.com>

* Update BTCPayServer/Services/LanguageService.cs

Co-authored-by: Andrew Camilleri <evilkukka@gmail.com>

* Update BTCPayServer/Services/LanguageService.cs

Co-authored-by: Andrew Camilleri <evilkukka@gmail.com>

* Update BTCPayServer/Services/LanguageService.cs

Co-authored-by: Andrew Camilleri <evilkukka@gmail.com>

* Added loop for all locales in Accept-Language sorted by weight + check if know this language

* New public method so a unit test can be created for it

* Unit test for language detection

* Fix language service when not in browser context

* fall back to default lang

* Auto-detect setting + ?lang=auto support

* Added invoice param "?lang=auto" info to docs

* Using null-coalescing assignment operator

* Reduce complexity and http dependency in language service

Co-authored-by: britttttk <39231115+britttttk@users.noreply.github.com>
Co-authored-by: Andrew Camilleri <evilkukka@gmail.com>

BTCPayServer/Models/StoreViewModels/CheckoutExperienceViewModel.cs

@@ -55,8 +55,10 @@ namespace BTCPayServer.Models.StoreViewModels
         [Display(Name = "Recommended fee confirmation target blocks")]
         [Range(1, double.PositiveInfinity)]
         public int RecommendedFeeBlockTarget { get; set; }
-
-
+        
+        [Display(Name = "Auto-detect language on checkout")]
+        public bool AutoDetectLanguage { get; set; }
+        
         [Display(Name = "Default language on checkout")]
         public string DefaultLang { get; set; }