select cast(replace('12,5',',','.') as float)
select cast(replace('12.5',',','.') as float)

select cast(replace('2.012,5',',','.') as float)
select cast(replace('2,012.5',',','.') as float)

SELECT @Fuelunits =
CASE
    WHEN ISNUMERIC(TRY_PARSE(@Hoeveelheid AS decimal(9,2) USING 'nl-NL')) = 1 Then CAST(TRY_PARSE(@Hoeveelheid AS decimal(9,2) USING 'nl-NL') AS FLOAT)
    ELSE CAST(TRY_PARSE(@Hoeveelheid AS decimal(9,2) USING 'en-US') AS FLOAT)
END