See the documentation here: https://docs.mendix.com/refguide8/parse-and-format-decimal-function-calls#1-introduction
If you use the following input:
formatDecimal(1234.56, '#,###.#')
the output is (depending on the language settings):
'1,234.5' or '1.234,5'
So the output is different depending in your are English or German.
Regards,
Ronald