A recent Bangkok immigration bureau circular for the first time advised those seeking a soft-power DTV extension to show photographs or videos of themselves learning Thai boxing or advancing their cookery skills in the nominated academy.
So I've just emailed the embassy in the Netherlands asking them if language school is also accepted. They replied with an pdf regarding all the visa's they have. The DTV one did not say language course unforteneley. So for now i would not take the risk and try to get an DTV based on this type of softpower.