In Catan seafarers expansion, when playing the road building card, can a settlement be built between building a free road and building a free ship?
No, it cannot. From the official Catan FAQ:
Development Cards – Progress – Road Building - May I use a Road Building card to build 1 road and 1 ship?
Yes. You must build them one after another though. You are not allowed to first build a road via a Road Building card, then build a settlement adjacent to that road by paying the required resources, and finally use the Road Building card once again, this time to build a ship.
The original German (I've the first German edition) uses the word sofort meaning immediately so from that I'd say no. Later and American/English versions may differ.
However, under what circumstances can you not immediately follow the placing of the two roads with placing a settlement between them, within the same turn?