feat: improve lang system to improve SEO

[mesqueeb]

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

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