Fix#954, which went stale 6 years ago and which I have now reopened. The `setLocale` method in `Calculation` and `Settings` does not affect the 3 localizable properties in `StringHelper` - `currencyCode`, `decimalSeparator`, and `thousandsSeparator`. One way to work around this problem is to set those properties separately; this is probably how most people handle it. Another way is to call Php's own `setlocale` function; this does not require any foreknowledge of what the values need to be, but it comes with baggage (data is maintained at process level rather than thread level), so its use is discouraged.
This PR adds a new `setLocale` method to `StringHelper`. It sets the 3 properties and the `Calculation` language all at once. It depends on the `Intl` extension, which is a recommendation but not a formal requirement for PhpSpreadsheet; the method will return `false` if `Intl` is not available or it thinks the supplied locale is not valid.
oleibman