Pass an explicit language to cart warnings and loyalty point messages

[KrzysztofPajak]

Part of the explicit-context work in Business (after #930, #931, #932, #936, #937)
Type: feature

Issue

57 calls in src/Business translated with ITranslationService.GetResource(name), which reads the working language of the current request. These were the 11 cart validators in Grand.Business.Checkout/Validators, the three loyalty point handlers for orders, and the loyalty points for registration. Outside a storefront request the text was wrong or missing. With no request (a scheduled task, a background handler) GetResource(name) returns an empty string. From the admin panel or the API it uses the admin's or caller's language, not the customer's.

Examples:

• An order marked as paid by a scheduled task or an admin wrote the loyalty points history entry ("Earned for order …") in the admin's language, or left it empty, instead of the customer's language.
• EndAuctionsTask validated the winning bid with empty warning texts, so the log line said nothing.
• The cart merge at login validated each item in the request language, which it took from the ambient context.

Solution

• Cart validation. IShoppingCartValidator methods and the validator records take a Language. The validators call GetResource(name, language.Id). IShoppingCartService.AddToCart, UpdateShoppingCartItem and MigrateShoppingCart, AddRequiredProductsCommand and ReOrderCommand pass the language down. Storefront and admin callers pass WorkContext.WorkingLanguage. PlaceOrderCommandHandler uses the language the order is placed in (PlaceOrderContainer.WorkingLanguage). EndAuctionsTask uses the admin language (LanguageSettings.DefaultAdminLanguageId), because its warnings only reach the log.
• Login and registration events. CustomerLoggedInEvent and CustomerRegisteredEvent carry the working language of the request. The login event's language goes to the cart merge, the registration event's language to the registration loyalty points. IExternalAuthenticationService.Authenticate and IOidcCustomerResolver.ResolveForLogin take it from the controllers (Facebook, Google, OpenID Connect). An account created from an API access token (OidcCustomerResolver.Resolve) has no request language. The registration handler then uses the store's default language, or the first language of the store.
• Loyalty points for orders. Award, reduce and return use Order.CustomerLanguageId, the language the customer placed the order in. This is a separate commit.
• Fallback language. The new extension ILanguageService.GetLanguageOrStoreDefault(languageId, store) returns the given language while it still exists. Otherwise it returns the store's default language, then the store's first language, then any language. The loyalty point handlers use it for an order without CustomerLanguageId or whose language or store was deleted. Before, such an order wrote the raw resource key or threw on a null key. The registration handler uses it when the event has no language, and EndAuctionsTask for the admin language. When no language exists at all, EndAuctionsTask logs a warning and skips the bid.
• Warning events. ShoppingCartItemWarningsEvent and ShoppingCartWarningsEvent carry the language, so plugins that add cart warnings can write them in the same language as the core warnings.

After this PR, src/Business has no GetResource(name) calls. That overload, and IPermissionService.Authorize(Permission), stay for Web, modules and plugins (the remainder documented in #937).

Breaking changes

• IShoppingCartValidator: every method takes a Language language. It is the last parameter, except in GetShoppingCartItemAttributeWarnings (before ignoreNonCombinableAttributes) and GetShoppingCartWarnings (after context).
• The ShoppingCart*ValidatorRecord records have a trailing Language Language.
• IShoppingCartService.AddToCart(customer, language, productId, shoppingCartType, storeId, warehouseId = null, …), UpdateShoppingCartItem(customer, language, shoppingCartItemId, warehouseId, attributes, …) and MigrateShoppingCart(fromCustomer, toCustomer, storeId, language, includeCouponCodes). The language comes right after the customer, so an old positional call does not compile instead of binding a null to the language.
• ShoppingCartItemWarningsEvent and ShoppingCartWarningsEvent take a trailing Language language (Language property).
• AddRequiredProductsCommand.Language and ReOrderCommand.Language (required).
• CustomerLoggedInEvent(customer, previousCustomer, storeId, language) and CustomerRegisteredEvent(customer, language). A null language means the registration had no request.
• IExternalAuthenticationService.Authenticate(parameters, customer, store, language, returnUrl = null). The protected ExternalAuthenticationService helpers take the language too.
• IOidcCustomerResolver.ResolveForLogin(sub, email, emailVerified, storeId, language).
• Constructors: CustomerRegisteredEventHandler, AwardLoyaltyPointsCommandHandler, ReduceLoyaltyPointsCommandHandler and ReturnBackRedeemedLoyaltyPointsCommandHandler add ILanguageService and IStoreService. EndAuctionsTask adds ILanguageService.

Testing

Automated: the solution builds, and dotnet test GrandNode.slnx passes (22 projects). New or changed tests cover:

• a validator writing its warning in the language it is given;
• ShoppingCartValidator passing the language into every record it builds;
• AddRequiredProductsCommand passing its language to AddToCart;
• the three loyalty point handlers translating in Order.CustomerLanguageId;
• the registration handler using the event's language, then the store's default language, then the first store language;
• the login handler passing the event's store and language to MigrateShoppingCart;
• the external authentication and OpenID Connect sign-in putting the request language on both events;
• EndAuctionsTask validating in the admin language.

Manual: run on 2026-10-17 with Playwright against a fresh install with sample data. A second language, "Polski", has every English resource prefixed with PL: , so each message shows which language it was written in. Loyalty points: 10 for registration, 1 point per 1 USD, awarded at Processing.

#	Scenario	Result
1	Register in Polish	Loyalty history: "PL: Registered as customer"
2a	Add a gift voucher to the cart without recipient details, in Polish	["PL: Enter valid recipient name","PL: Enter valid recipient email"]
2b	Update the cart quantity to 100000, in Polish	"PL: The maximum quantity allowed for purchase is 100."
3	Place an order in Polish (cash on delivery), then "Mark as paid" from the admin panel in English	"PL: Earned promotion for order #1": the order's language, not the admin's
4	A guest adds a product in Polish, then logs in	The product is in the account's cart
5	Control in English: registration and the gift voucher	Messages without PL:

The server log showed no errors other than e-mail sending (no SMTP server locally).

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