import re
s = "Mein ,Name, ist John, Wo23 bist+ ,_du? , MeinName "
replacements = { "Mein": "my", "ist": "is", "Wo":"where", "bist":"are", "du":"you", "is" :"iis"}
rx = r'(?:{})(?=[,/!?()_0-9\-=+."\s\'])'.format('|'.join(map(re.escape, replacements.keys())))
print (rx)
print ( re.sub(rx, lambda m: replacements[m.group()], s) )
#my ,name,is John,where23 are+,_you? ,MeinName