I discovered last night that in French a cog is a une dent, which also means a tooth, or une dent d’engrenage (“tooth gear”), and a cog wheel is une roue dentée (a toothed wheel), which is kind of a cog looks like.
The English word cog, meaning a tooth on a gear, or a gear or a cogwheel, comes from the Middle English cogge, from the Old Norse kugg (notch), from the Proto-Germanic *kuggō (cog, notch), from the Proto-Indo-European *gugā (hump, ball), from *gēu- (to bend, arch).
A cog can also refer to an unimportant individual in a greater system, e.g. He’s just a cog in the machine, which in French would be Il n’est pas qu’un rouage de la machine – rouage is another word for cog or gearwheel, and also means part. Les rouages means machinery, as in les rouages de l’État (the machinery of state) or les rouages de l’administration (the wheels of government).
In German a cog is Zahn (tooth) and a cogwheel is Zahnrad (toothwheel). He is only a cog in a machine is Er ist nur ein Rädchen im Getriebe (“He is only a little wheel in the works/gears/gearbox”), or Er ist nur eine Nummer unter vielen (“He is only a number among many”).
Are there similar expressions in other languages about being a cog in a machine?