CaoimhínSF wrote:
What Niall says is basically true, but I saw this handled very well in a movie a few years ago, where only some characters were speaking Irish, and the approach they took could work in writing as well. As an example, in the movie a man was making a claim against an elderly woman in front of a judge (the details are unimportant), and the woman answered a question from him with Níl [a] fhios agam ("I don't know"). The man turned angrily to the judge (it was an informal court) and said something like "How can she say that she doesn't know? She knows very well." The idea was that the judge understood her perfectly, and this was not intended as a translation for the judge's benefit, but as furthering his complaint. It was a clever way to incidentally translate her words for the viewer, and since it was set in a time when many people still understood Irish, but were switching to speaking English, it made sense that he was speaking English himself.
Your situation would be somewhat different, since it would not make sense for any of the people truly to be speaking English (English was largely in its infancy then, at least at the beginning of your period), and you will presumably have to pick some other language as the "norm" (the language your English text represents). In the period 600-900, at least four languages were being spoken in what is now Scotland.
In the Hebrides, the west coast, and parts of the southwest, Gaelic had been introduced a few centuries before (or possibly earlier than that) by conquerors/settlers form Ireland. In other parts of the southwest, the people were still speaking a Brythonic/Cumbric language, related to (or possibly much the same as) the ancestor of modern Welsh, which is believed to have survived there for several more centuries, before ultimately being overwhelmed by Gaelic.
In the northeast, people were still speaking Pictish, which scholars now believe (after a lot of controversy) was also a Brythonic language. Pictish is believed to have died out soon after the end of your period, as the Gaelic and Pictish kingdoms merged (caused at least partially by the Viking invasions which disrupted their societies) and Gaelic then replaced Pictish in its homeland. If your story brings in the Vikings, then Norse would be a fifth language to take into account.
In the extreme southeast, Anglo-Saxon speakers started to settle during your period, and what later came to be called Scots (a Scottish relative of English) ultimately developed out of their speech. Even they were, however, taken over by Gaelic to some extent towards the end of your period, as the Gaelic kingdom continued to expand. The people in the southeast never became fully Gaelic-speaking, although it did become the language of government there, and many if not most of them would probably have learned it, at least to some extent, as a second language. After only 2-3 centuries, though, Gaelic started to decline there, and Scots began spreading out into the rest of the Lowlands, ultimately gaining the support of the monarchy and replacing Gaelic as the language of government, and spreading gradually throughout the Lowlands in a process that ended in some places only the 19th century, when English began spreading through the Highlands as well. In the later stages of this process, "true" Scots eventually lost out to more standard English, and few people in Scotland can actually speak real Scots any more.
Since your book is going to be largely in English, you will presumably have to choose one of those language groups to be the main one, and have them speak English instead of their true language, but that will give you the flexibility to play around with the others and have them speak their various languages, and having people translate for one another will make sense historically for that period, since they didn't all start speaking Gaelic at once. You could choose the Brythonic speakers as the "true" natives, and have people translate what the Gaelic speakers say, or vice versa. Little distinctively Pictish vocabulary is really known, so you won't be able to use their language much (except for place names), but you could also make them the "English" speakers, and then have people translating from both Gaelic and the southwestern Cumbric into "Pictish/English".
Now Fraoch, don't you wish you were just writing another book about vampires!

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Is foghlaimeoir mé. I am a learner. DEFINITELY wait for others to confirm and/or improve.
Beatha teanga í a labhairt.