df = pd.DataFrame({'a': ['120,00', '42,00', '18,00', '23,00'], 'b': [
    '51,23', '18,45', '28,90', '133,00']})
df['a'] = [x.replace(',', '.') for x in df['a']]
df['a'] = df['a'].astype(float)
Convert commas to dots within a Dataframe