If you don't know the customs (does this include laws, rules of hospitality, dress, etc.? Customs encapsulate almost everything that makes a society distinct and thus identifiable), the language, or where you are then how do you know you're in the twelfth-century?

What knowledge do you actually have of the Middle Ages which could help you? Would you recognise the difference between a Lithuanian or a Danish church? Do you know enough about plants or animals to be able to identify which are particular to one region? Could you identify any geographical landmarks? If you can't do any of these things (and I doubt you'd be able to read the scripts of the period, not impossible but certainly difficult if you don't speak the languages) then you're stuffed.

We could drop you at Stonehenge and you'd probably be able to work out where you are, or Rome. Knowing when you are helps, but unless you, for example, know medieval heraldry (does that not count as custom?) then working out who Richard the Lionheart is would be difficult.

Ultimately this scenario requires you to have the perfect knowledge and be put in position to use it immediately.