This issue occurs when a locale is set that uses comma as decimal separator (e.g. locale.setlocale(locale.LC_NUMERIC, 'de_DE.utf8')).
I have a tkinter.Spinbox with increment=0.1 connected to a tkinter.DoubleVar.
When I change the value of the Spinbox using the arrow buttons and subsequently try to read out the variable with tkinter.DoubleVar.get(), my code throws the follwoing error:
_tkinter.TclError: expected floating-point number but got "0,1".

Here is a minimal code example:

-------------
import tkinter
import locale

locale.setlocale(locale.LC_NUMERIC, 'de_DE.utf8')


class TestDoubleVar():
    def __init__(self):
        root = tkinter.Tk()
        self.var = tkinter.DoubleVar()
        self.var.set(0.8)
        number = tkinter.Spinbox(
            root,
            from_=0, to=1, increment=0.1,
            textvariable=self.var,
            command=self.update,
            width=4
        )
        number.pack(side=tkinter.LEFT)
        root.mainloop()

    def update(self, *args):
        print(float(self.var.get()))


if __name__ == '__main__':
    TestDoubleVar()

-------

Actual result: the code throws an error

Expected result: the code should print the values of the DoubleVar even with a locale set that uses comma as the decimal separator.

n.b. the problem also occurs with tkinter.Scale

I just found the following code in lines 314-320 of [tkinter/ttk.py](https://github.com/python/cpython/blob/master/Lib/tkinter/ttk.py), which are clearly not localization-aware:
```
def _to_number(x):
    if isinstance(x, str):
        if '.' in x:
            x = float(x)
        else:
            x = int(x)
    return x
```

I'll keep looking for similar stuff and add a pull request once I think I have nailed down the issue
I'll look for something similar in

Is this a problem exclusive to tkinter? I ran the following code, and it appears to be an issue with the builtin 'float' method:

>>> import locale
>>> locale.setlocale(locale.LC_NUMERIC, 'de_DE.utf8')
'de_DE.utf8'
>>> float("1,2")
ValueError: could not convert string to float: '1,2'

I have somehow managed to corrupt my locales so I would appreciate if you could try on your machine as well (is it just me?!).

I wrote a longer explanation but BPO and Chrome ate it.

The issue is that DoubleVar is not locale aware. I don't know what should be the actual fix but the following may be a useful workaround (It works on my system):

import tkinter
import locale
import sys

locale.setlocale(locale.LC_NUMERIC, 'de_DE')
class Var(tkinter.DoubleVar):
    def get(self):
        return locale.atof(self._tk.globalgetvar(self._name))

class TestDoubleVar():
    def __init__(self):
        root = tkinter.Tk()
        self.var = Var()
        self.var.set(0.8)
        number = tkinter.Spinbox(
            root,
            from_=0, to=1, increment=0.1,
            textvariable=self.var,
            command=self.update,
            width=4
        )
        number.pack(side=tkinter.LEFT)
        root.mainloop()
    def update(self, *args):
        print(self.var.get())
if __name__ == '__main__':
    TestDoubleVar()

See also issue12558 which exposes the other side of the same issue.

Las time I looked at it, it was inherently a Tcl issue. In some places it uses locale-dependent formatting of floating point numbers, but locale-unaware parsing, or vice versa.