the l10n get function for content scripts.

content-script.js

function _(id) {
  // I'm using self here, but replace it with addon, in case you're loading your content script directly in your html file for a panel
  return new Promise((resolve) => {
    self.port.once("translated", resolve);
  });
  self.port.emit("translate", id);
}

// Example usage:
_("anerror").then((string) => {
  console.log(string);
});

main.js

"use strict";

const { get: _ } = require("sdk/l10n");
const { PageMod } = require("sdk/page-mod");

const pageMod = new PageMod({
  // Your page-mod config
});

// Your worker/port thing is saved in pageMod
pageMod.port.on("translate", (id) => {
  pageMod.port.emit("translated", _(id));
});