ki18n: add locale-aware date formatting via FormatTime

Adds FormatTime(ctx, t) for locale-aware date output (e.g. "12 mai 2026"
in fr-FR). Init now accepts extra Locale values so callers can register
additional languages with custom formatters; en-GB and fr-FR are built-in
defaults.

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

2 files changed, 113 insertions, 3 deletions

diff --git a/ki18n/ki18n.go b/ki18n/ki18n.go
index 8f20ad4..b517d06 100644
--- a/ki18n/ki18n.go
+++ b/ki18n/ki18n.go
@@ -2,8 +2,10 @@ package ki18n
 
 import (
 	"context"
+	"fmt"
 	"io/fs"
 	"net/http"
+	"time"
 
 	"github.com/a-h/templ"
 	"github.com/kataras/i18n"
@@ -64,6 +66,42 @@ func LangMiddleware(strategies ...Strategy) func(http.Handler) http.Handler {
 	}
 }
 
+var frenchMonths = [...]string{
+	"janvier", "février", "mars", "avril", "mai", "juin",
+	"juillet", "août", "septembre", "octobre", "novembre", "décembre",
+}
+
+// Locale configures a language for use with ki18n. Lang is a BCP-47 tag (e.g. "es-ES").
+// FormatTime is optional; if nil, en-GB formatting is used.
+type Locale struct {
+	Lang       string
+	FormatTime func(time.Time) string
+}
+
+var defaultLocales = []Locale{
+	{
+		Lang:       "en-GB",
+		FormatTime: func(t time.Time) string { return t.Format("2 January 2006") },
+	},
+	{
+		Lang: "fr-FR",
+		FormatTime: func(t time.Time) string {
+			return fmt.Sprintf("%d %s %d", t.Day(), frenchMonths[t.Month()-1], t.Year())
+		},
+	},
+}
+
+var timeFormatters map[string]func(time.Time) string
+
+func FormatTime(ctx context.Context, t time.Time) string {
+	lang, _ := ctx.Value(contextKey{}).(string)
+	f, ok := timeFormatters[lang]
+	if !ok {
+		f = timeFormatters[defaultLang]
+	}
+	return f(t)
+}
+
 func Tr(ctx context.Context, format string, args ...any) templ.Component {
 	lang, _ := ctx.Value(contextKey{}).(string)
 	if lang == "" {
@@ -76,12 +114,25 @@ func Tr(ctx context.Context, format string, args ...any) templ.Component {
 	return templ.Raw(msg)
 }
 
-func Init(fs fs.FS) error {
-	loader, err := i18n.FS(fs, "*/*.yml")
+func Init(localesFS fs.FS, extra ...Locale) error {
+	all := append(defaultLocales, extra...)
+
+	langs := make([]string, len(all))
+	timeFormatters = make(map[string]func(time.Time) string, len(all))
+	for i, loc := range all {
+		langs[i] = loc.Lang
+		if loc.FormatTime != nil {
+			timeFormatters[loc.Lang] = loc.FormatTime
+		} else {
+			timeFormatters[loc.Lang] = timeFormatters[defaultLang]
+		}
+	}
+
+	loader, err := i18n.FS(localesFS, "*/*.yml")
 	if err != nil {
 		return err
 	}
-	inst, err := i18n.New(loader, "en-GB", "fr-FR")
+	inst, err := i18n.New(loader, langs...)
 	if err != nil {
 		return err
 	}
diff --git a/ki18n/ki18n_test.go b/ki18n/ki18n_test.go
index fa1f226..5956be5 100644
--- a/ki18n/ki18n_test.go
+++ b/ki18n/ki18n_test.go
@@ -9,6 +9,7 @@ import (
 	"strings"
 	"testing"
 	"testing/fstest"
+	"time"
 
 	"github.com/a-h/templ"
 	"github.com/martinlehoux/kagamigo/kcore"
@@ -54,6 +55,64 @@ func TestTr_withoutLangInContext(t *testing.T) {
 	assert.Equal(t, "Hello", result)
 }
 
+func TestFormatTime_englishLocale(t *testing.T) {
+	ctx := context.WithValue(context.Background(), contextKey{}, "en-GB")
+	d := time.Date(2026, time.May, 12, 0, 0, 0, 0, time.UTC)
+
+	assert.Equal(t, "12 May 2026", FormatTime(ctx, d))
+}
+
+func TestFormatTime_frenchLocale(t *testing.T) {
+	ctx := context.WithValue(context.Background(), contextKey{}, "fr-FR")
+	d := time.Date(2026, time.May, 12, 0, 0, 0, 0, time.UTC)
+
+	assert.Equal(t, "12 mai 2026", FormatTime(ctx, d))
+}
+
+func TestFormatTime_unknownLocale(t *testing.T) {
+	ctx := context.WithValue(context.Background(), contextKey{}, "de-DE")
+	d := time.Date(2026, time.May, 12, 0, 0, 0, 0, time.UTC)
+
+	assert.Equal(t, "12 May 2026", FormatTime(ctx, d))
+}
+
+func TestFormatTime_extraLocale(t *testing.T) {
+	spanishMonths := [...]string{
+		"enero", "febrero", "marzo", "abril", "mayo", "junio",
+		"julio", "agosto", "septiembre", "octubre", "noviembre", "diciembre",
+	}
+	fs := fstest.MapFS{
+		"es-ES/index.yml": &fstest.MapFile{Data: []byte{}},
+	}
+	kcore.Expect(Init(fs, Locale{
+		Lang: "es-ES",
+		FormatTime: func(t time.Time) string {
+			return fmt.Sprintf("%d de %s de %d", t.Day(), spanishMonths[t.Month()-1], t.Year())
+		},
+	}), "failed to init fs")
+	ctx := context.WithValue(context.Background(), contextKey{}, "es-ES")
+	d := time.Date(2026, time.May, 12, 0, 0, 0, 0, time.UTC)
+
+	assert.Equal(t, "12 de mayo de 2026", FormatTime(ctx, d))
+}
+
+func TestFormatTime_extraLocaleNoFormatter(t *testing.T) {
+	fs := fstest.MapFS{
+		"es-ES/index.yml": &fstest.MapFile{Data: []byte{}},
+	}
+	kcore.Expect(Init(fs, Locale{Lang: "es-ES"}), "failed to init fs")
+	ctx := context.WithValue(context.Background(), contextKey{}, "es-ES")
+	d := time.Date(2026, time.May, 12, 0, 0, 0, 0, time.UTC)
+
+	assert.Equal(t, "12 May 2026", FormatTime(ctx, d))
+}
+
+func TestFormatTime_noLocaleInContext(t *testing.T) {
+	d := time.Date(2026, time.May, 12, 0, 0, 0, 0, time.UTC)
+
+	assert.Equal(t, "12 May 2026", FormatTime(context.Background(), d))
+}
+
 func TestCookieStrategy_found(t *testing.T) {
 	r := httptest.NewRequest(http.MethodGet, "/", nil)
 	r.AddCookie(&http.Cookie{Name: "lang", Value: "fr-FR"})