`cal` does try to use the expected first day of the week based on the locale. It seems to at least use LC_TIME from some quick local testing:

  bratch@serenity ~ $ LC_TIME=en_GB cal | head -n2
        May 2017      
  Mo Tu We Th Fr Sa Su
  
  bratch@serenity ~ $ LC_TIME=en_US cal | head -n2
        May 2017      
  Su Mo Tu We Th Fr Sa