ki18n: derive language from request context via strategy chain

Replace Tr(lang, ...) with Tr(ctx, ...) — language is now resolved once
per request by LangMiddleware and stored in context. Callers no longer
need to thread a lang string through to templates.

Adds Strategy interface with CookieStrategy and AcceptLanguageStrategy
built-ins; middleware chains them in order, falling back to en-GB.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

1 files changed, 90 insertions, 3 deletions

diff --git a/ki18n/ki18n_test.go b/ki18n/ki18n_test.go
index d2e7e16..fa1f226 100644
--- a/ki18n/ki18n_test.go
+++ b/ki18n/ki18n_test.go
@@ -4,6 +4,8 @@ import (
 	"bytes"
 	"context"
 	"fmt"
+	"net/http"
+	"net/http/httptest"
 	"strings"
 	"testing"
 	"testing/fstest"
@@ -17,8 +19,9 @@ func TestSimpleText(t *testing.T) {
 	initFrenchLocale(map[string]string{
 		"Hello you": "Bonjour toi",
 	})
+	ctx := context.WithValue(context.Background(), contextKey{}, "fr-FR")
 
-	result := render(t, Tr("fr-FR", "Hello you"))
+	result := render(t, Tr(ctx, "Hello you"))
 	assert.Equal(t, "Bonjour toi", result)
 }
 
@@ -26,8 +29,9 @@ func TestVariables(t *testing.T) {
 	initFrenchLocale(map[string]string{
 		"Hello %s": "Bonjour %s",
 	})
+	ctx := context.WithValue(context.Background(), contextKey{}, "fr-FR")
 
-	result := render(t, Tr("fr-FR", "Hello %s", "John"))
+	result := render(t, Tr(ctx, "Hello %s", "John"))
 	assert.Equal(t, "Bonjour John", result)
 }
 
@@ -35,11 +39,89 @@ func TestSpanClass(t *testing.T) {
 	initFrenchLocale(map[string]string{
 		"Hello <span class=\"text-bold\">%s</span>": "Bonjour <span class=\"text-bold\">%s</span>",
 	})
+	ctx := context.WithValue(context.Background(), contextKey{}, "fr-FR")
 
-	result := render(t, Tr("fr-FR", "Hello <span class=\"text-bold\">%s</span>", "John"))
+	result := render(t, Tr(ctx, "Hello <span class=\"text-bold\">%s</span>", "John"))
 	assert.Equal(t, "Bonjour <span class=\"text-bold\">John</span>", result)
 }
 
+func TestTr_withoutLangInContext(t *testing.T) {
+	initFrenchLocale(map[string]string{
+		"Hello": "Bonjour",
+	})
+
+	result := render(t, Tr(context.Background(), "Hello"))
+	assert.Equal(t, "Hello", result)
+}
+
+func TestCookieStrategy_found(t *testing.T) {
+	r := httptest.NewRequest(http.MethodGet, "/", nil)
+	r.AddCookie(&http.Cookie{Name: "lang", Value: "fr-FR"})
+
+	s := CookieStrategy("lang")
+	assert.Equal(t, "fr-FR", s.Detect(r))
+}
+
+func TestCookieStrategy_missing(t *testing.T) {
+	r := httptest.NewRequest(http.MethodGet, "/", nil)
+
+	s := CookieStrategy("lang")
+	assert.Equal(t, "", s.Detect(r))
+}
+
+func TestAcceptLanguageStrategy_found(t *testing.T) {
+	initFrenchLocale(map[string]string{})
+	r := httptest.NewRequest(http.MethodGet, "/", nil)
+	r.Header.Set("Accept-Language", "fr-FR,fr;q=0.9")
+
+	assert.Equal(t, "fr-FR", AcceptLanguageStrategy.Detect(r))
+}
+
+func TestAcceptLanguageStrategy_missing(t *testing.T) {
+	initFrenchLocale(map[string]string{})
+	r := httptest.NewRequest(http.MethodGet, "/", nil)
+
+	assert.Equal(t, "", AcceptLanguageStrategy.Detect(r))
+}
+
+func TestLangMiddleware_firstStrategyWins(t *testing.T) {
+	first := strategyFunc(func(_ *http.Request) string { return "fr-FR" })
+	second := strategyFunc(func(_ *http.Request) string { return "de-DE" })
+
+	var capturedLang string
+	handler := LangMiddleware(first, second)(http.HandlerFunc(func(_ http.ResponseWriter, r *http.Request) {
+		capturedLang, _ = r.Context().Value(contextKey{}).(string)
+	}))
+
+	handler.ServeHTTP(httptest.NewRecorder(), httptest.NewRequest(http.MethodGet, "/", nil))
+	assert.Equal(t, "fr-FR", capturedLang)
+}
+
+func TestLangMiddleware_fallsThrough(t *testing.T) {
+	first := strategyFunc(func(_ *http.Request) string { return "" })
+	second := strategyFunc(func(_ *http.Request) string { return "fr-FR" })
+
+	var capturedLang string
+	handler := LangMiddleware(first, second)(http.HandlerFunc(func(_ http.ResponseWriter, r *http.Request) {
+		capturedLang, _ = r.Context().Value(contextKey{}).(string)
+	}))
+
+	handler.ServeHTTP(httptest.NewRecorder(), httptest.NewRequest(http.MethodGet, "/", nil))
+	assert.Equal(t, "fr-FR", capturedLang)
+}
+
+func TestLangMiddleware_fallback(t *testing.T) {
+	first := strategyFunc(func(_ *http.Request) string { return "" })
+
+	var capturedLang string
+	handler := LangMiddleware(first)(http.HandlerFunc(func(_ http.ResponseWriter, r *http.Request) {
+		capturedLang, _ = r.Context().Value(contextKey{}).(string)
+	}))
+
+	handler.ServeHTTP(httptest.NewRecorder(), httptest.NewRequest(http.MethodGet, "/", nil))
+	assert.Equal(t, "en-GB", capturedLang)
+}
+
 // render is a test helper that renders a component and returns the result as a string
 func render(t *testing.T, c templ.Component) string {
 	t.Helper()
@@ -63,3 +145,8 @@ func initFrenchLocale(translations map[string]string) {
 	}
 	kcore.Expect(Init(fs), "failed to init fs")
 }
+
+// strategyFunc is a test helper to create a Strategy from a plain function
+type strategyFunc func(*http.Request) string
+
+func (f strategyFunc) Detect(r *http.Request) string { return f(r) }