setting a locale that uses comma as decimal separator breaks tkinter.DoubleVar

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-03-02.15:11:30.139>
labels = ['3.7', 'type-bug', 'expert-tkinter']
title = 'setting a locale that uses comma as decimal separator breaks tkinter.DoubleVar'
updated_at = <Date 2021-08-21.08:08:14.008>
user = 'https://github.com/thawn'

bugs.python.org fields:

activity = <Date 2021-08-21.08:08:14.008>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Tkinter']
creation = <Date 2020-03-02.15:11:30.139>
creator = 'thawn'
dependencies = []
files = ['48943']
hgrepos = []
issue_num = 39827
keywords = []
message_count = 5.0
messages = ['363184', '363371', '371394', '400005', '400023']
nosy_count = 4.0
nosy_names = ['serhiy.storchaka', 'thawn', 'epaine', 'andrei.avk']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue39827'
versions = ['Python 3.7']