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The setup of the thousands separator is located under: Settings > Translations > Languages.
For each language, you can set a different separator format (and few other formatting options like the dates format or the text direction).
3 options modify the format of a number:
- Separator format: number of digits between separators
- Thousands Separator: the character displayed at each separation
- Decimal Separator: character displayed before the decimals
If you use [3,3,3,-1], the number 1232345.01 will be displayed 1,232,345.01. However, the number 1237891232345.01 will be displayed 1237'891'232'345.01.
The -1 at the end of the list means 'stop the separation'. The value 0 can be used to mean 'repeat the last group at infinite'.
So the correct form for a separator between each 3 digits whatever the number of digits is: [3, 0].
Reference: Olivier Dony's comment
Under Settings > Languages > Your favorite language you will find two fields: thousands separator which is , in English by default, but to make it work you also need to set the Separator Format. [3,3,3,-1] will make sure e.g. that 1232345.01 EUR appears as 1,232,345.01 EUR. The tooltip of the Separator Format will show you that the separator is fully customizable. Apparently, some countries prefer 4 instead of 3 digits for separation.