Do I render and return salute to the soldiers(officers) of other countries?

Yes.

Always, unless they are an enemy combatant.

I had several foreign deployments in Latin America and was pretty familiar with their rank structure. In El Salvador I had a gold star over my pocket which was the equivalent of a US Army Major so the Salvadoran soldiers would recognize my rank.

there are so many variables ! at SHAPE, no saluting so as not to attract attention from observers .

Depends on their rules and requirements and location . at SHAPE (international) there was no saluting at all so as to not attract 'enemy' attention to the rank of a person .