import re
string = '1. 0.1, 0.2, тест, тест. 2. тест, тест, 0.5=0.3+0.2.'
new = re.sub(r'(?<=\d)\.(?=\d)', ',', string)
print(new)
aW1wb3J0IHJlCnN0cmluZyA9ICcxLiAwLjEsIDAuMiwg0YLQtdGB0YIsINGC0LXRgdGCLiAyLiDRgtC10YHRgiwg0YLQtdGB0YIsIDAuNT0wLjMrMC4yLicKbmV3ID0gcmUuc3ViKHInKD88PVxkKVwuKD89XGQpJywgJywnLCBzdHJpbmcpCnByaW50KG5ldykK
1. 0,1, 0,2, тест, тест. 2. тест, тест, 0,5=0,3+0,2.