The FORMAT function does not support the Invariant culture
The FORMAT function appears to not support the Invariant culture.
The documentation for the FORMAT function reads that the 'culture' argument "accepts any culture supported by the .NET Framework as an argument". The .NET Framework supports the Invariant culture, and it is documented that the identifier of that culture is an empty string (as opposed to null).
So this call is supposed to return number 42 formatted according to the Invariant culture:
select format(42, '0.0', '')
Instead it throws an exception, "The culture parameter '' provided in the function call is not supported.", which contradicts the documentation that tells all .NET cultures are supported.
This issue exists from the very beginning when the FORMAT function was introduced.
StackOverflow question regarding this issue: https://dba.stackexchange.com/q/48289/5203