When I write, I care about my word choice.  When I read the two words, I perceive different connotations, so I would not use them interchangeably in my writing.

Suitable means usable for a purpose.

Appropriate means acceptable for a purpose.

Eg.  The dress is appropriate for the occasion but is not suitable for me.

If you say, the dress is suitable for the occasion but is not appropriate for me, then it doesn't quite make sense.

So appropriate tends to be a measure against some standards or expectations.

I agree - they are very similar. The main difference is that suitable could be a matter of taste or preference. For example, a roast beef sandwich would not be a suitable lunch for someone who is vegetarian, but would be suitable for other people. Usually, appropriate implies more of an objective criteria. For example, it is appropriate to dress neatly in a suit if you are a man going to an interview, but it is not appropriate to wear a T-shirt.

Pavel,

in short they mean the same thing, in most social situations you can use either. In a more long answer, suitable means something is okay for a certain circumstance (such as that car is suitable for this trip) whereas appropriate tends to describe something non offensive (such as saying please and thank you). I hope this helps you. If you have further questions don't hesitate to message me. Good luck!