The developers of Castle Game Engine have introduced a ready-made Steam integration into the engine. Now you can use the Steam API to get some information, manage achievements, etc. The integration only requires the Steam DLL/SO from Valve, nothing else.
Works on all supported platforms: Windows, Linux, macOS. Works with all supported compilers: FPC, Delphi. Ready-made Pascal CastleSteam module. Also supplied is a ready-made example in the engine sources: examples/steam.