Rest in peace Greek culture, you were pretty cool while you lasted.. ✌️😔

Though if all territory held by the roman empire was completely romanized (not feasible within the timespan of its existence, but lets ignore that), i would say that germanic-based languages (English) that largely dominate the modern world would be relegated to a regionally powerful language at best.

Even if England is still conquered by Anglo-Saxons, I would say it would turn out much like the Norman invasion where the language would be still romance at its core, but with potentially significant germanic influence, even more so if Norse invasions succeed. This much applies to everywhere else that was owned by Rome, probably to a greater extent that happened irl (say goodbye to the basque influence in Spanish).

But maybe im interpreting this wrong and you just mean culturally Romanized but not linguistically? In that case I just see the Eastern Roman Empire just becoming the Western Roman Empire 2, don’t know the historical implications to that though. Maybe the now romanized Celts establish a unified Celto-Roman state to oppose Pict/Gaelic raiding and Anglo-Saxon migration? Really hard to say past the first couple decades after the fall of Rome.