translation: add fallback option to langMeta

If set to a language code (e.g fr-fr), any missing strings will be
filled in from that language (if possible) rather than from the default
en-us. Currently not used, but could be useful in the future for
variations of the same language.

lang.go

@@ -2,6 +2,8 @@ package main
 
 type langMeta struct {
 	Name string `json:"name"`
+	// Language to fall back on if strings are missing. Defaults to en-us.
+	Fallback string `json:"fallback,omitempty"`
 }
 
 type quantityString struct {