the classical way to pluralize "–al" words:
un animal → des animaux [en: animal(s)]
un journal → des journaux [en: journal(s)]
with some exceptions: un carnaval → des carnavals [en: carnival(s)]
un festival → des festivals [en: festival(s)]
un idéal → des idéals (OR des idéaux) [en: ideal(s)]
un val → des vals (OR des vaux) [en: valley(s)]
There is no logic there (as many things in french), it's up to Mistral to choose how the plural can beEDIT: Format + better examples