Hi Martin,
After a bit of a search, it turns out that Splunk provides for this, however just not in a pretty GUI.
Have a look here: http://docs.splunk.com/Documentation/Splunk/latest/Developer/TranslateSplunk#Localize_dates_and_numbers
Hope this helps 🙂
EDIT: Just as I clicked away, I think I just got what you were after. What if I had a file that was created by a system that used a decimal comma for localisation and I wanted to index & report on that along with files created in places that used a decimal point. Being able to specify the number localisation by host/source/sourcetype would be quite useful for this kind of scenario.
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