Since I am one in favor of localization in the most extent possible,
here is my contribute (not the first time, we talk about this).

Leaving the GUI aside (considering that solved), I think that we could
have a directive (or environmental variable setting) that a script
author would add in the script. The omission of that setting would go
for EN (as it is now).

But to guarantee that the setting defined by the script's author is
kept in the script, the setting would be explicit. For example, jut to
see the idea:
#define decimal_dot_thousands_comma
#define decimal_comma_thousands_dot
set number_style(',' , '.') (...)