Latin was the language of the church and law in the medieval period. Due to this Latin was the written language of nobility. These Scottish clans heritage was not necessarily rooted in the Gaelic as the lines of lordship were greatly reset after 1066 and this had both direct and indirect effect that stretched from England all the way north into Scotland. This inevitably sparked a key theme of early to mid-Medieval Scotland which was the drawing of lines between the central and lowlands of Scotland and the western Highlands and Islands of Scotland. The western isles and highlands had a much more concrete Gaelic heritage and as the chivalric culture of medieval Europe expanded into the noble classes in the central lowlands. This clash of cultures and family heritages eventually caused much friction and inter clan violence through the years. Before long though the dominant chivalric culture and governance of medieval Europe won out over the Gaelic influence in western Scotland. As such though some families maintained the Gaelic culture as seen in the houses with Gaelic mottos, the majority adopted the dominant traditions and language of European nobility. They used Latin mottos because of this, practically these mottoes had to be read and understood by of all of nobility throughout Europe and their class.

To add to this question; clan graham has a motto of "do not forget" but I can't seem to find concrete info about the source of this motto. Is anyone familiar with this area of Scottish history that could share their knowledge?

How far back can we date the clan mottos and crests? Is it anything like tartans which I have heard are more of a 19th century aristocratic invention?

Do the latin mottos reflect clans where leadership actually left the Highlands? For example, the Lord Reay of Clan MacKay has been Dutch for at least the past two centuries.