Is it ok doing a tongue biopsy while having a few severly decayed teeth?or does this increase the risk of infection of the biopsy site?

Oral Bacteria: Your mouth will always be contaminated, even if you restore or extract those teeth. If your oral surgeon isn't concerned you should be fine.

Safety: If surgeon deems biopsy to be necessary, do not wait. The sooner you have procedure accomplished the sooner future treatment can be started. As to the severely decayed (infected ) teeth, what are you waiting for? Have the infection felt with immediately. Procrastination is not your friend.