feat: add ability to set locale

[bachya]

Describe what the PR does:

SSIA. This is useful when a device/gateway reports values with different decimal representations (e.g., 47,123 instead of 47.123). Setting the appropriate locale will allow these numbers to be parsed correctly.

Does this fix a specific issue?

Fixes https://github.com/bachya/ecowitt2mqtt/issues/946

Checklist:

• [ ] Confirm that one or more new tests are written for the new functionality.
• [x] Run tests and ensure everything passes (with 100% test coverage).
• [x] Update README.md with any new documentation.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (69a1ce5) to head (fd935cf).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #1014 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 32 32 Lines 1761 1775 +14 ========================================= + Hits 1761 1775 +14 ```

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