feat(i18n): Fallback to messages from 'und' language if no localized

classes/base.lua

@@ -45,6 +45,7 @@ class.packages = {}
 
 function class:_init (options)
   if self == options then options = {} end
+  SILE.languageSupport.loadLanguage('und') -- preload for unlocalized fallbacks
   self:declareOptions()
   self:registerRawHandlers()
   self:registerCommands()

core/languages.lua

@@ -65,6 +65,12 @@ SILE.registerCommand("fluent", function (options, content)
     message = entry:format(options)
   else
     SU.warn(string.format("No localized message for %s found in locale %s", key, locale))
+    fluent:set_locale('und')
+    entry = fluent:get_message(key)
+    if entry then
+      message = entry:format(options)
+    end
+    fluent:set_locale(locale)
   end
   SILE.processString(("<sile>%s</sile>"):format(message), "xml")
 end, nil, nil, true)

i18n/und.ftl

@@ -1 +1,3 @@
 hello = Namárië <em>{ $name }</em>!
+
+tableofcontents-not-generated = Rerun SILE to process the table of contents!