fonttools/Lib/fontTools/feaLib/testdata/language_required.fea

languagesystem latn DEU;
languagesystem latn FRA;
languagesystem latn ITA;

feature hlig {
  script latn;
  language DEU exclude_dflt required;
  sub D E U by D_E_U;

  language FRA exclude_dflt;
  sub F R A by F_R_A;
} hlig;

feature liga {
  language ITA exclude_dflt required;
  sub I T A by I_T_A;
} liga;

feature scmp {
  sub [a-z] by [A.sc-Z.sc];
} scmp;