UCI is very different from the Chess Engine Communication Protocol. PolyGlot is a UCI-to-xboard adapter developed by Fabien. It connects a UCI chess engine to an xboard interface such as WinBoard. UCI2WB is another such adapter (for Windows).
Standalone PolyGlot can be used, along with Fruit, as chess engine for chess frontends, suchs as XBoard. In that case, PolyGlot and Fruit run as two independent single-threaded processes. PolyGlot starts first, and it forks Fruit. Both processes get communicated by by means of pipes: PolyGlot captures Fruit's standrad input and output.
PolyGlot tries to solve known problems with other adapters. For instance, it detects and reports draws by fifty-move rule, repetition, etc.
PolyGlot 1.4 has been adapted and incorporated to GNU Chess v6 as chess engine protocol adapter. It connects Fruit-based GNU Chess engine to the good old GNU Chess frontend.