feat(i18n): add completeness test and fix missing JP strings

TestLangCompleteness uses reflection to walk the i18n struct and fail
on any empty string field, catching incomplete translations at CI time.
Running it immediately found three missing strings in lang_jp.go
(playtime, timer.enabled, timer.disabled), which are now filled in.
CHANGELOG updated with i18n refactor, FR/ES languages, and the new test.

server/channelserver/sys_language_test.go

@@ -1,6 +1,8 @@
 package channelserver
 
 import (
+	"fmt"
+	"reflect"
 	"testing"
 
 	cfg "erupe-ce/config"
@@ -92,3 +94,36 @@ func TestGetLangStrings_EmptyLanguage(t *testing.T) {
 		t.Errorf("Empty language should default to English, got %q", lang.language)
 	}
 }
+
+// checkNoEmptyStrings recursively walks v and fails the test for any empty string field.
+func checkNoEmptyStrings(t *testing.T, v reflect.Value, path string) {
+	t.Helper()
+	switch v.Kind() {
+	case reflect.String:
+		if v.String() == "" {
+			t.Errorf("missing translation: %s is empty", path)
+		}
+	case reflect.Struct:
+		for i := 0; i < v.NumField(); i++ {
+			checkNoEmptyStrings(t, v.Field(i), path+"."+v.Type().Field(i).Name)
+		}
+	case reflect.Slice:
+		for i := 0; i < v.Len(); i++ {
+			checkNoEmptyStrings(t, v.Index(i), fmt.Sprintf("%s[%d]", path, i))
+		}
+	}
+}
+
+func TestLangCompleteness(t *testing.T) {
+	languages := map[string]i18n{
+		"en": langEnglish(),
+		"jp": langJapanese(),
+		"fr": langFrench(),
+		"es": langSpanish(),
+	}
+	for code, lang := range languages {
+		t.Run(code, func(t *testing.T) {
+			checkNoEmptyStrings(t, reflect.ValueOf(lang), code)
+		})
+	}
+}