This usage of 'supposed to', though common enough in English today, is poorly treated in the three big English dictionaries I just consulted. eg https://dictionary.cambridge.org/dictionary/english/supposed
Here is my try. To translate it as a legal or moral duty seems to detract from the speculative aspect of a supposition. I think it is used loosely for different purposes. It looks like an impersonal, but if rephrased to 'it was supposed that I would study languages', you can ask, 'Well, who supposed it?'. Yourself, teachers, university rules,etc? Then it is an expectation on your future behaviour, based on some reasons not currently mentioned. Someone's supposition. I think this is a better approach to 'supposed to', rather than the shorthand for a duty as 'have to'.
A second meaning, as 'assumed', is mentioned here too https://slovniky.lingea.cz/anglicko-cesky/supposed
There are subtle meanings in English which are not necessarily the same for "was supposed to" and "should have". Randonneur3 covers "supposed to" nicely, but "should have", apart from those meanings, can also have the additional meaning that the speaker regrets not having taken the action described. For instance, in this case, "I should have studied languages" can mean the speaker realising in later life that knowledge of languages would have been of benefit to him or her. Only the context of the statement would indicate which meaning was intended. I wondered if the Czech statement would cover all these nuances, or whether some of them would have to be covered in a different way? So to summarise, we have 1. failure to meet an obligation 2. failure to meet an assumption 3. regret at not having done something.