[chrispenycate]

In what I consider "mediaeval", English did not yet exist as a language. It was coming into being, but was essentially two streams. the Romance-based upper-class Norman, and the germanic, peasant Anglo-Saxon (plus all the church Latin, educated Greek, invading Scandinavian et cetera that went to produce the modern unwieldy bastard we know and love) Thus, what you'd get as word order and even speech rhythms would depend on the social stratum from which she originated. Read some Chaucer in the original. But, if thou art minded to use the second person singular, recall that it was used to inferiors, family members and close friends, with the plural "you" for those whom thou respecteth, or towards whom thou wouldst seem polite.
Moreover, without travel, nor radio, nor contact much twixt any save the rich, regional differences were more marked than now – a tinker would effectively speak several different tongues within a day, close relatives but differing in all, vocabulary, vowel stress, nor vowel colour would not pass across, and oft would speaking Latin with the priest be simpler than comprehending simple folk. (Swiss german, swiitzeteuch, between the isolated valleys shows much the same fragmentation today)
The nobles visit, they exchange their kin across the county lines, and thus their speech is more homogenous, but even then the travel's hardships mean they ofttimes bide at home. Still, they can write, and learnèd tutors can be brought to set their tongue to courtly usage.
Would I could thee further aid, but whilst I dabble in the art of tongues, I am no specialist to criticise nor guide.