code: def _inject_language(self, src, strings):
      
      if src not in self.sources:
        raise ValueError("Invalid source for '{0}': {1}".format(self.name, src))
      def _format_string(s):
        if "{0}" in s and "{1}" and "{src}" in s:
          return s.format(*sorted([src, self.target]), src=src)
        elif "{0}" in s and "{1}" in s:
          return s.format(*sorted([src, self.target]))
        elif "{src}" in s:
          return s.format(src=src)
        else:
          return s
      return [_format_string(s) for s in strings]
docs: Injects languages into (potentially) template strings.