So, bought sime nice leather sandals 3 months ago. I wore them since, and my feet was sweaty in it a couple of times that I suspect now causes this bad smell. Should I put the thing in the washing machine? :)
No you should not put them in the washing machine.
Use saddle soap. Work it into a lather and apply to the leather, wiping it off as you go. Try not to use a lot of water as you do this. If you don't have saddle soap use something mild like Ivory or Dove.
Warning - leather cleaners can change the appearance of leather.
Also check with the manufacturer. Most manufacturers provide such conservative instructions that they are useless but you might get lucky.