The admission for the course is enough to apply for VISA, but weather you will get it depends on subjective decision of an official. But I would suggest you to apply for a course which returns money in case VISA was not granted. For example here they say:
The fee might be returnable unless an applicant is unable to get a visa to study at the Centre (after providing a proper document proving this fact).
thinkpoland.org/kursy_jezykowe_i_studia/132,one_year_polish_course_.html