As of version 4.2 (https://jira.atlassian.com/browse/JRA-14076) JIRA honors user preferences whenever issue is edited. So if you want comma to be accepted - you need to use locale which accepts comma (like German or Polish) - either per user in his profile or globally for whole JIRA setting default locale (applied unless the user selected his own locale). If you want to use decimal dot, you would need to apply locale using dot - e.g. English. From the source code I don't see any option to support both at the same time for single user.
BTW: For banking industry that would be quite unsafe as "," and "." are both used and they do mean something different (thousands separator vs. decimal separator).
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