Update README.md

[7777Hecker]

add the new language: Vietnamese

Describe your change

Insert a breif description about what your change is / does.

If this is related to an issue, please link the issue here

New feature checklist

• Have you run the application to make sure your code runs
• Is this not just a simple formatting change

Translation checklist

• Have you run the application to make sure your code runs
• Have you used the correct keys for your translations added (e.g. en)
• Have you made sure to leave a , at the end of each translation added
• Have you added the language to supportedLanguages in alphabetical order
• Have you added the language to the table in README.md in alphabetical order
• If you have translated the README, have you linked it in README.md

[brentvollebregt]

There are no translations in this PR - maybe you forgot to add them?

Nevermind, I now see they are in #390 - could you merge the changes in the two PRs into one of these PRs? (either is fine).

[brentvollebregt]

I cherry-picked this PR into #390