You can use Weblate for the translation.
Then, Weblate will automatically make commits once a day under your username with your translations.
You'll be able to test one of the development versions after each commit from Weblate.

The alternative way is to modify directly the .ts file in /translations.
Even if you choose this way, you should at least start on Weblate so that the file is automatically created with the correct name and tag.
You can then modify the .ts file of your language (in /translations) if you want to test the translation by compiling MPC-QT yourself or forking MPC-QT and pushing commits to your fork.
This is a good way to start the translation by using an LLM (also called "AI"). Make sure you check the translation as they tend to make obvious errors.
Then you can make a pull request with the mpc-qt_en.ts renamed to the correct name for your language (and change the language tag inside).
The vanished strings don't show up in Weblate because they're old string (meaning not being used anymore in MPC-QT).
They're kept so that translators don't have to retranslate them in case they're used again in the future, so you don't have to translate them, you can safely ignore them.

Q: How can I change the UI language?

A: MPC-QT uses the language of your system if available. You can force it to use English in the options.