After a while, the Visigoths began speaking what became Spanish. The Lombards adopted an Italian dialect. The Franks stopped speaking Old Franconian (which went on to develop into Dutch) and started speaking Old French. But this did not happen in Britain. There, the invading Anglo-Saxons brought their language and imposed it upon the locals, who adapted so completely to speaking Old English dialects that fewer than a half dozen words were borrowed from what became Old Welsh. Just a fluke? Or is there something special about English?
Even Scots, which is a variety of English, survived to become the main language of Scotland. The Northumbrians conquered the British peoples around the Firth of Forth, the area today known as the Lowlands. The British and Pictish peoples adopted the Anglian dialect as their own and kept it even after the dismemberment of Northumbria by the Vikings, and then gave it equal footing with Gaelic in the formation of the Kingdom of Scotland under Kenneth Mac Alpin and his successors. The suppression of Scots Gaelic under the Hanoverians cannot explain the hardihood of English in the 9th and 10th Centuries beyond England's borders.
When the Normans conquered England in 1066, they imposed Norman French upon the country in much the same way other conquerors had imposed their languages upon subject peoples. And like those other conquerors, they eventually began speaking the language of those they had conquered, and English re-emerged as the national language. So why didn't the Anglo-Saxons adopt Old Welsh in the first place? What is there about English that makes it a hardier plant with deeper roots than other languages?
Note that I'm saying nothing here about English's later spread as the language of colonists and imperial administrators. English in the early modern period and even today plays a role similar to Latin and Greek in the Classical period and later. But even in its earliest days, English seems to be a special case among languages.
The language of educated people in the Old English period was Latin, and if you knew Latin you could still converse with other educated people all over the old Empire. This situation continued for many centuries. And, of course, that meant that church services were conducted in Latin everywhere. Yet the Anglo-Saxons seem to have wanted the gospel in their own language and prayers in their own language more than other peoples. The English, from the beginning, have led the way in Bible translation and the creation of native language catechesis. King Alfred was quite fluent in Latin, but he wanted all his people to acquire the treasures of learning available to them, so he started a project of translating major works of philosophy and so on into the English of his day. He himself took the lead in translating Boethius's Consolation of Philosophy.
Either there's something special about English, or there's something special about how the English felt about their language. Either way, it fascinates me.