Bruce and King Edward would have spoken Norman French when speaking to each other but it is generally considered that the likes of Bruce and Wallace were not monolingual. Bruce was seemingly also at least a Gaelic speaker too - and Gaelic and Scots were the dominant spoken languages in Scotland at that time. Written communication in Scotland at that time was in both Latin and Norman-French so both those were in use. The Gaelic language predominated in the Highlands and part of the Lowlands north of the Tay plus in the western Southern Uplands. Professor A.A.M Duncan in his "Scotland The Making Of A Kingdom" yes concedes that Norman French was used by the Anglo-Normans but also states that "it seems generally agreed that from the borders of Galloway to the Mounth and east of the Highland Line the landowning class from laird to earl, spoke Scots, a version of northern English, by the late 13thC" In Galloway itself, which at that time included the Bruce's Earldom of Carrick, Prof Duncan goes on to state the dominance of Gaelic in the mid 13thC among the ruling elite is evidenced by the charters of Duncan, Earl of Carrick.

Which brings us to the idea of Bruce not being Scottish. We all know he was part Anglo-Norman and that his family owned land both in Scotland and England and that the family had a history of swerving allegiances - but he was also through his mother descended from said Duncan mentioned above and the native Gaelic population as well as of course the other Scottish links re the Royal household etc. Yes he was an Anglo-Norman but he also had a foot in the Scottish and the Gaelic world. The historian Fiona Watson was frustrated at the way the UK media jumped on her point that Bruce was probably born on his family's Essex estate and that this made him English and not Scottish. She did not assert that at all and regards that as more an accident of birth and suggests his upbringing and experience were more relevant than his place of birth. Little is known about Bruce's actual upbringing but Watson asserts that as was the tradition in Gaelic Scotland at that time he was fostered to another family. His younger brother Edward seemingly to the O'Neil's in Ulster but she believes the evidence suggests Bruce more likely spent 7 years in a Hebridean household possibly the MacDonalds of Islay as stated in her new bio on Bruce by Watson "Traitor Outlaw King Part 1". This is a cut and paste from her own website

"This seems to me rather more important when it comes to understanding the future king than his Essex connections, which don’t tell us anything we didn’t already know about the way he was brought up. But learning Gaelic and absorbing the rather different culture – up to and including the way to wage war – of the Irish Sea world surely made a considerable difference to young Robert’s outlook."

So considering that to be authentic the film would need to show Bruce speaking two, possibly more than two, languages - plus the Scots in general would need to be speaking Gaelic or early Scots - then it is going to need to be a subtitled film which does not suit everyone's taste. If Bruce is shown as speaking English dialogue then an American accent or English accent doesn't make any more sense than a Scottish accent which most folk I imagine can relate to him having in the film.

As to in Mary's time and the idea of English being the language of most of mainland Britain. I don't see the relevance in that statement. Mary was Queen of Scots not Queen of Most Of Mainland Britain, and the primary languages in Scotland in that time were Scots (the Scottish form of English) and Gaelic. Scots was the language of law and the language of the Royal Court. What was spoken elsewhere in mainland Britain isn't really that relevant. She would have spoken to her subjects and nobles in her own Scots. We don't really know how heavy a French accent her Scots would have had. Again if you want the dialogue to be authentic then yes the actors could all have learned to speak in 16thC Scots and some in Gaelic - and the lead actress playing Mary, when not speaking French, could have given her 16thC Scots a bit of a French accent. It would again for an international audience be subtitled. Didn't it just make much more sense just to have modern Scottish accents to show they were Scottish characters??