Dazbo, I think the Anglo-Saxons must have arrived in large numbers including women and family groups because if the menfolk had mainly taken Britons as wives the Celtic languages and other influences would have been stronger. It was the women who mainly reared the kids and taught them their language. I don't say that the entire Anglo-Saxon people left Europe but that the nature of their 'invasion' must have been something more than a purely military campaign. They were invited in after all, by the British ruling class, in a misguided attempt to protect themselves.

It is pretty certain, as I understand it, that the Roman legions that occupied Britain were from many parts of the Roman empire. Higher officers would have been required to have good Latin and middle-ranking officers would surely have been bilingual to a greater or lesser extent according to their responsibilities, but just as the soldiers of the Highland Scottish, Indian and Gurkha regiments of the British army often had little or no English, the common Legionnaire would have had no such need beyond a few words of command.

In my view a soldier would have been as likely to have picked up just as much of the local language sufficient for what was needed to do basic commercial transactions. As for a requirement to have Latin to be a Roman citizen, given the administrative task of checking it I doubt that more than basic communication could have been expected. Even today in immigrant societies such requirements are kept to basics and standards are often dependent upon the zeal of the examiner. Given the demand for a loyal population I daresay a retired legionnaire out on the frontier of empire, who applied for his citizenship and gave it as his intention to settle in a newly conquered territory with his local wife and kids, would have been given a wide margin for error in his Latin test.