chore(languages): Warn when setting a language that has no been loaded

core/languages.lua

@@ -10,17 +10,17 @@ loadkit.register("ftl", function (file)
    return assert(fluent:add_messages(contents))
 end)
 
-local loadonce = {}
+SILE.scratch.loaded_languages = {}
 
 SILE.languageSupport = {
    languages = {},
    loadLanguage = function (language)
       language = language or SILE.settings:get("document.language")
       language = cldr.locales[language] and language or "und"
-      if loadonce[language] then
+      if SILE.scratch.loaded_languages[language] then
          return
       end
-      loadonce[language] = true
+      SILE.scratch.loaded_languages[language] = true
       local langresource = string.format("languages.%s", language)
       local gotlang, lang = pcall(require, langresource)
       if not gotlang then

core/settings.lua

@@ -20,6 +20,13 @@ function settings:_init ()
       type = "string",
       default = "en",
       hook = function (language)
+         if SILE.scratch.loaded_languages and not SILE.scratch.loaded_languages[language] then
+            SU.warn(([[Setting document.language to '%s', but support for '%s' has not been loaded!
+
+  Consider invoking \language[main=%s] which loads language support before
+  setting it or manually calling SILE.languageSupport.loadLanguage("%s").
+            ]]):format(language, language, language, language))
+         end
          fluent:set_locale(language)
       end,
       help = "Locale for localized language support",