feat: improve lang system to improve SEO

current built files, built with `presite` include initial English text for SEO purposes.

- [ ] need to update the way Japanese text is shown so it's included in the built files at `.presite/`
- [ ] probably possible by rendering every string of text in the DOM and hide/show the correct one with `v-show`
- [ ] double check result with `npm run build`

PS: Since we're already live, any new ticket is to be branched from `production` with branch name `pr-` and make a PR towards `production` again.
Also, remember we're using `npm` again now (as opposed to `yarn`).