Check that fallback_language exists between locales building static bundles

[mondeja]

Raise the next compilation error if fallback_language not exists in locales directory:

error: Fallback language not found in locales directory
 --> path/to/file.rs:8:28
  |
8 |         fallback_language: "en-US",
  |                            ^^^^^^^

[XAMPPRocky]

Thank you for your PR, and congrats on your first contribution! 🎉