see https://github.com/kataras/iris/issues/1591#issuecomment-674473247

i18n/i18n.go

@@ -406,7 +406,7 @@ func (i *I18n) GetMessage(ctx *context.Context, format string, args ...interface
 }
 
 // Wrapper returns a new router wrapper.
-// The result function can be passed on `Application.WrapRouter`.
+// The result function can be passed on `Application.WrapRouter/AddRouterWrapper`.
 // It compares the path prefix for translated language and
 // local redirects the requested path with the selected (from the path) language to the router.
 //
@@ -417,7 +417,10 @@ func (i *I18n) Wrapper() router.WrapperFunc {
 	}
 	return func(w http.ResponseWriter, r *http.Request, next http.HandlerFunc) {
 		found := false
-		path := r.URL.Path[1:]
+		path := r.URL.Path
+		if len(path) > 0 && path[0] == '/' {
+			path = path[1:]
+		}
 
 		if idx := strings.IndexByte(path, '/'); idx > 0 {
 			path = path[:idx]
@@ -451,7 +454,6 @@ func (i *I18n) Wrapper() router.WrapperFunc {
 					}
 				}
 			}
-
 		}
 
 		next(w, r)

i18n/loader.go

@@ -172,6 +172,21 @@ func (l MemoryLocalizer) GetLocale(index int) context.Locale {
 	// 	panic(fmt.Sprintf("locale of index [%d] not found", index))
 	// }
 	// return loc
+	/* Note(@kataras): the following is allowed as a language index can be higher
+	than the length of the locale files.
+	if index >= len(l) || index < 0 {
+		// 1. language exists in the caller but was not found in files.
+		// 2. language exists in both files and caller but the actual
+		// languages are two, while the registered are 4 (when missing files),
+		// that happens when Strict option is false.
+		// force to the default language but what is the default language if the language index is greater than this length?
+	 	// That's why it's allowed.
+		index = 0
+	}*/
+
+	if index < 0 {
+		index = 0
+	}
 
 	return l[index]
 }