NumberFormatTransformer uses a NumberFormatter which relies on the system locale. This makes it impossible to use NumberFormatTransformer with data whose locale does not match the system locale.
TestNumberFormatTransformer fails on some locales for similar reasons because the grouping symbol differs in different locales.
This issue adds a locale attribute for NumberFormatTransformer which allows the user to specify the locale which should be used for formatting. The locale must be specified as land-country e.g. en-US