lang: add a way to check language availability

gamecore/lang.lua

@@ -72,6 +72,18 @@ function LanguageManager:getCurrentLangName()
   return self:getLangName(self.data.current)
 end
 
+function LanguageManager:isLangAvailable(lang)
+  local isAvailable = false
+
+  for i,v in ipairs(self.data.available) do
+    if v == lang then
+      isAvailable = true
+    end
+  end
+
+  return isAvailable
+end
+
 -- TRANSLATION FUNCTIONS
 -- get the translation of a string