Exactly. This. If you stick to this principle, you really can't go wrong. It works better than to try to reproduce exact speech patterns. After all, you can write about somebody speaking Russian without writing IN Russian. Same principle. Just use ordinary non-slangy English. Watch out for anachronisms, but keep to the topics the person would have been talking about and don't worry about reproducing their actual speech. You are writing in modern English, after all. Concentrate on reproducing what they would have been thinking as they spoke, rather than on how they would have phrased the speech.
1066 in this year the monastery at Westminster was hallowed on Childermas Day (28 December). And King Eadward died on Twelfth-Mass Eve (5 January) and he was buried on Twelfth Mass Day, in the newly hallowed church at Westminster. And Earl Harold succeeded to the Kingdom of England, as the king had granted it to him and men had also chosen him thereto and he was blessed as King on twelfth-mass day. And in the same year that he was king he went out with a naval force against William ... and the while count William landed at Hastings, on St.Michael's Mass Day and Harold came from the north and fought against him before his army had all come and there he fell and his two brothers Gyrth and Leofwine and William subdued this land, and came to Westminster and Archbishop Ealdred hallowed him king and paid him tribute and gave him hostages and afterwards bought their land. Found that via Google Books
Bernard Cornwell has written a series of stories set in exactly that period in British history: https://en.wikipedia.org/wiki/The_Saxon_Stories His use of modern English but without glaring anachronisms would do well as a template for your efforts. It's also worth reading the series if you want to avoid plowing the same ground as he did.
Yes. A few years ago I had the great sadness of beta-reading a novella set in southern Scotland in the 1100s. In it, random members of the gentry drop in for tours of the local laird's castle/Stately Home, and afterwards are invited to drink tea (!!!) in the "drawing room." The heroine is a rich merchant's daughter who, when she isn't working her fingers to the bone in her father's household because they have no servants inside or out, dreams of marrying the laird's son and doing Art for Art's Sake. And so it went. I tried to be gentle with the writer, even linking to Internet articles on the subjects (s)he as getting so anachronistically wrong . . . until I read the ending, where, not in an Afterword but in the last chapter text itself, s(he) bragged about how well-researched and historically accurate her/his novella was. That's when I had to write, "No, sorry. It's not. Not by a long shot." ( "Nooooooooooo!!!!!") I didn't hear from the writer after that, and it's just as well. @Jared-Johannson, you do not want to be that author. You want to write the kind of historical fiction that high school students taking their SATs can get answers from, even if they didn't do that much with Anglo-Saxon history in school.
Nice translation. If you read it aloud, and allow for word order, it is actually almost understandable. The 7 appears to be punctuation, a period or comma. An. M.LXVI. On þyssum geare man halgode þet mynster æt Westmynstre on Cyldamæsse dæg... In this year, one hallowed that monastery (that monastery was hallowed, construction still used in German) at Westminster on Childmas day
Cornwell's stuff is pretty good I think. He uses old names for places/people and throws in various insults and phrases that I assume he made up. You could also read Beowulf and the various Norse sagas. The way they structure a story is interesting and could give you some thoughts. They usually begin by setting out who someone is - their antecedents, etc and also throw in stuff about how people relate to each other in society.
Basically, I find it best if the language used in the dialogue is fairly transparent - the reader shouldn't have to think too hard about your choices, so modern/neutral is probably a good way to go (no overt 'historical' usage but no glaringly modern either). The devil is in the details, so a good book on usage/sayings/proverbs (so you can check when a particular, ye-olde-worldly-sounding phrase actually first started in usage - you'd be surprised how recent many of them are) wouldn't hurt. You can flavor the dialogue (for a different period, Georgette Heyer did a splendid job of making dialogue feel period and yet utterly accessible) but it's quite difficult, especially with the period you've chosen, and the danger is in making it too heavy. Oddly, being very authentic can often make it sound more fake.
I think it pretty much is German. I remember my German ex was reading Old English primary sources for his history essays at uni and had no problem with comprehension because it was that close to German. This pretty much gave him an advantage as most English students simply couldn't access those books.
I studied Old English for a bit at university and it takes me ages to read it. One trick to add a bit realism would be to find recurring idioms and turns of phrase in Old English and adopt those, though in modern English, obviously.
Go and read Beowulf, if you haven't before, to get an idea for the phrasings and word choices. "LO, praise of the prowess of people-kings of spear-armed Danes, in days long sped, we have heard, and what honor the athelings won! Oft Scyld the Scefing from squadroned foes, from many a tribe, the mead-bench tore, awing the earls. Since erst he lay friendless, a foundling, fate repaid him: for he waxed under welkin, in wealth he throve, till before him the folk, both far and near, who house by the whale-path, heard his mandate, gave him gifts: a good king he! To him an heir was afterward born, a son in his halls, whom heaven sent to favor the folk, feeling their woe that erst they had lacked an earl for leader so long a while; the Lord endowed him, the Wielder of Wonder, with world's renown. Famed was this Beowulf:[1] far flew the boast of him, son of Scyld, in the Scandian lands."
As has been said many times before in this thread, I do not think it necessary to use proper 8th century english. In terms of History, "Old English" is not really a proper language but more akin to a mixture of multiple languages and dialects into something vaguely coherant and 'lingua franca'. Additionally, spelling of particular words at the time was not an established practice and neither was grammar rules - and the mixture of languages and dialects did not help. "Old English" in 8th century wessex is vastly different to "Old English" in 8th century Anglia. Many words did not exist like "table" or "road" whilst others did like "Knifr" [Knife]. It is a common misconception that "Old English" was a coherant single language similar to what we now call French or German nowadays but it is more a melting pot of mostly Gealic (Breton, Cornish, Scottish, etc...), Danish (Jutes), Frisian, Dutch, Saxon, Welsh and French (not Frankish) with a substantial influence of various germanic dialects and Latin. It also perhaps has a hint of other languages like Spanish or ancient Greek. However, there are many "Core" words that people agree that most areas that spoke "old english" understood or used with a fairly similar definition. However, the extent and usage of those words is debated. Point is, it would be very difficult to do this even with extensive research in the particular location, time and context of the speech being given to whom and by what kind of person of which nationality and background. Of note, "Old English" covers a period of language that extends almost 700 years by some reckonings. So really, it it quite a broad subject.