Tip trouvé sur planet ubuntu-fr :

# cat /usr/share/i18n/SUPPORTED | 
    grep fr_FR >> /var/lib/locales/supported.d/fr
# locale-gen
Generating locales...
  en_AU.UTF-8... done
  en_BW.UTF-8... done
  en_CA.UTF-8... done
  en_DK.UTF-8... done
  en_GB.UTF-8... done
  en_HK.UTF-8... done
  en_IE.UTF-8... done
  en_IN.UTF-8... done
  en_NZ.UTF-8... done
  en_PH.UTF-8... done
  en_SG.UTF-8... done
  en_US.UTF-8... done
  en_ZA.UTF-8... done
  en_ZW.UTF-8... done
  fr_BE.UTF-8... up-to-date
  fr_CA.UTF-8... up-to-date
  fr_CH.UTF-8... up-to-date
  fr_FR.ISO-8859-15@euro... done
  fr_FR.ISO-8859-1... done
  fr_FR.UTF-8... up-to-date
  fr_LU.UTF-8... up-to-date
Generation complete.

Bien utile après une mise à jour de son système…