ci: add validation for localization

[mbedded]

There should be some kind of checks regarding the localization.
For example:

• Check if all "Localization JSON files" have the same keys/same structure. Use EN as the default.
• Check if the used localization is existing. The code uses t("my-key") so the files can be read and compared with the existing JSON files.
• Check if there are keys, which are never used.

We aim to make the plugin more robust and find such issues early during development or when PRs are being created.