If Pierre is telling the truth then Qadr is not telling the truth. However, this means that Ratna is telling the truth, so this leads to a contradiction as Pierre stated that just one person is telling the truth.

So Pierre is not telling the truth, which means that Qadr is telling the truth, but Ratna is not telling the truth. This in turn means that Sven is telling the truth, but Tanya is not.

So only Qadr and Sven are telling the truth.
See all short problems arranged by curriculum topic in the short problems collection
This problem is taken from the UKMT Mathematical Challenges.