Convert Python strings into floats explicitly using the comma or the point as separators

because I don’t know the locale settings

You could look that up using the locale module:

>>> locale.nl_langinfo(locale.RADIXCHAR)
'.'

or

>>> locale.localeconv()['decimal_point']
'.'

Using that, your code could become:

import locale
_locale_radix = locale.localeconv()['decimal_point']

def read_float_with_comma(num):
    if _locale_radix != '.':
        num = num.replace(_locale_radix, ".")
    return float(num)

Better still, the same module has a conversion function for you, called atof():

import locale

def read_float_with_comma(num):
    return locale.atof(num)