In bug report QTBUG-115740 the Spanish number localization received some updates how the CLDR has stated the thousand separators to work.

Could there be implemented an optional "useGrouping" which our user is requesting:
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I was still checking the topic on the web, and I come across a similar example in JS.

https://stackoverflow.com/questions/61705430/chrome-tolocalestring-thousands-separator-is-not-working-for-spanish

Although by default it is true that they do what the CLDR dictates, at least they have made it possible to apply the useGrouping option to obtain the expected value. From Qt, I think that at least this casuistry should be able to be contemplated, since as I mentioned previously, in Spain it is the most widespread format

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/NumberFormat/NumberFormat#usegrouping
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This kind of format is possible for example in Microsoft Excel, Google Sheets etc.