add: writing invalid decimal/digit group marks #2656

I'm not sure whether this is related or a different problem: with this journal file

commodity 1.000,00

account assets                                    ; type: C
account assets:Sparkasse:Giro
account expenses:Mitarbeiter:HinzKunz

2026-07-02 Gehalt
    assets:Sparkasse:Giro                      -1000
    expenses:Mitarbeiter:HinzKunz               1000

if a add a new "Gehalt" entry with hledger 1.99.3-g1a8fc8921-20260701, windows-x86_64 (I've only type "Ge<tab>" and then pressed Enter for the accounts and amounts):

To quit, enter . at a date prompt or press control-d or control-c.
Date [2026-07-02]:
Description: Gehalt
Using this similar transaction for defaults:
2026-07-02 Gehalt
    assets:Sparkasse:Giro                     -1.000,
    expenses:Mitarbeiter:HinzKunz              1.000,

Account 1 [assets:Sparkasse:Giro]:
Amount  1 [-1.000]:
Account 2 [expenses:Mitarbeiter:HinzKunz]:
Amount  2 [1.000]:
Account 3 (or . or enter to finish this transaction):
2026-07-02 Gehalt
    assets:Sparkasse:Giro                     -1.000.
    expenses:Mitarbeiter:HinzKunz              1.000.

Save this transaction to the journal ? [y]:
Saved.
Starting the next transaction (. or ctrl-D/ctrl-C to quit)

it will added in this format:

2026-07-02 Gehalt
    assets:Sparkasse:Giro                     -1.000.
    expenses:Mitarbeiter:HinzKunz              1.000.

which fails checking:

hledger.exe: Error: C:\temp\a\2026.journal:12:53:
   |
12 |     assets:Sparkasse:Giro                     -1.000.
   |                                                     ^
invalid number (invalid use of separator)

Originally posted by @tmssngr in #2645