Aside from the batch of 4 that was executed early in the war I don't believe any Mercs have been tried as such. Russia makes heavy use of Wagner and Kadyrovites so I doubt they want to get stuck in the tit for tat of prosecuting and executing mercs, the 4 that got hanged were charged and found guilty by the DPR government which has since been subsumed into the Russian government. There may be other cases, but I only remember the 4, one British two Moroccan and some other nation I cannot remember.