If the culture of the result doesn't matters or we're only talking of integer values, CONVERT or CAST will be fine.
However, if the result must match a specific culture, FORMAT might be the function to go:
DECLARE @value DECIMAL(19,4) = 1505.5698
SELECT CONVERT(NVARCHAR, @value) --> 1505.5698
SELECT FORMAT(@value, 'N2', 'en-us') --> 1,505.57
SELECT FORMAT(@value, 'N2', 'de-de') --> 1.505,57
For more information on FORMAT see here.
Of course, formatting the result should be a matter of the UI layer of the software.