// language = de, en, en-au, en-us, ...
static File gttsCLI(S text, S language, File mp3File) {
  assertTrue("ggts-cli not installed", hasGttsCLI());
  backtick("gtts-cli " + platformQuote(text) + " --lang " + pqO(language) + " --nocheck --output " + pqO(mp3File);
  ret mp3File;
}