import re
key = "äg"
value = "tät"
line = "dfäg,"
line = re.sub(ur'%s,' % key.decode('utf8'), ur'%s,' % value.decode('utf8'), line.decode('utf-8'), flags=re.UNICODE).encode("utf8")
print(line)
aW1wb3J0IHJlCmtleSA9ICLDpGciCnZhbHVlID0gInTDpHQiCmxpbmUgPSAiZGbDpGcsIgpsaW5lID0gcmUuc3ViKHVyJyVzLCcgJSBrZXkuZGVjb2RlKCd1dGY4JyksIHVyJyVzLCcgJSB2YWx1ZS5kZWNvZGUoJ3V0ZjgnKSwgbGluZS5kZWNvZGUoJ3V0Zi04JyksIGZsYWdzPXJlLlVOSUNPREUpLmVuY29kZSgidXRmOCIpCnByaW50KGxpbmUp