Vue-i18n localStorage.lang not working after building project

Question

I get a translated json-file with

const skillsJson = require("./locales/" + localStorage.lang + "/skills.json");

and it works perfectly fine when I run it with npm run serve.

But after building the project using npm run build I get this error in the console:

Uncaught Error: Cannot find module './undefined/skills.json'
    at i (skills\.json$:13)
    at s (skills\.json$:8)
    at Module.56d7 (App.vue:225)
    at r (bootstrap:79)
    at Object.0 (app.5886208a.js:1)
    at r (bootstrap:79)
    at n (bootstrap:45)
    at bootstrap:152
    at app.5886208a.js:1

Seems like the local language didn't get found and the fallback language didn't work. But I think my i18n.js file is fine. Here it is:

import Vue from "vue";
import VueI18n from "vue-i18n";

const moment = require("moment");

Vue.use(VueI18n);

function loadLocaleMessages() {
  const locales = require.context(
    "./locales",
    true,
    /[A-Za-z0-9-_,\s]+\.json$/i
  );
  const messages = {};
  locales.keys().forEach(key => {
    const matched = key.match(/([A-Za-z0-9-_]+)\./i);
    if (matched && matched.length > 1) {
      const locale = matched[1];
      messages[locale] = locales(key);
    }
  });
  return messages;
}
let lang = "de";
if (localStorage.getItem("lang")) {
  lang = localStorage.getItem("lang");
} else {
  localStorage.setItem("lang", lang);
  moment.locale(localStorage.getItem("lang"));
}

export default new VueI18n({
  locale: lang,
  fallbackLocale: process.env.VUE_APP_I18N_FALLBACK_LOCALE || "de",
  messages: loadLocaleMessages()
});

Any guesses on what could be the problem? If you need any additional informations tell me.