Not so. The Highland Scots spoke Gaelic. The historic common language of the Lowands however was Northern English - from the 7th century onwards brought by the Anglo-Saxons who arrived in Caledonia at almost exactly the same time as the Scots. It continued to be called English (often spelled Inglis and other variants) until the very end of the 15th century when some Scots, correctly or incorrectly,  began calling their northern variant(s) of English 'Scots'. They had previously called Gaelic 'Scots'. Scots-English (in addition to Scots) was certainly commonplace by the Act of Union in 1707 - and indeed was already commonplace by the time of the King James bible in 1611. Historic spelling sometimes gives a quite false impression of difference, but there was no real standard spelling anywhere in Britain, in England or Scotland, until the 18th century.  Today some believe that Scots was and is a different language from English, others that Scots is simply the collective name for the historic variants of English found in Scotland.