As far as as I know it's a general international recommendation to use a Thin space as thousands separator, also see
"for numbers with many digits the digits may be divided into groups of three by a thin space, in order to facilitate reading. Neither dots nor commas are inserted in the spaces between groups of three".
LibO should allow th use a thin space alternatively to different localization related digit group separators.
Activation in CALC could be by an additional checkbox in Menu 'Format -> Cells -> Numbers'.
In Writer a similar checkbox in the 'Tabe -> Number Format' dialog would be appropriate.
I can't tell whether other formatting dialogs have to be considered.
Making this NEW as it's confirmed and setting to lowest enhancement for priority. I may take this one in the coming weeks but until then if someone has the time, shouldn't take too long to do.
From a recent question in the Spanish Ask (https://ask.libreoffice.org/es/question/58628/espacio-fino-no-separable/) I have ended on the Institutional Style guide for the European Union
inside it specifically on the http://publications.europa.eu/code/en/en-360500.htm
In the Spanish version of the 6.5 page figure "thin space" not only space. I think in relation with the "The International System of Units (SI)"
where the use of thin space is indicated.
There is a NARROW NON-BREAK SPACE (Decimal: 8239, Hex: 202F) in the "General punctuation" subset of the fonts (http://www.alanwood.net/unicode/general_punctuation.html), that could be a good candidate for use as thin space.
Being defined in "The International System of Units (SI)" maybe an option, could be, to have it as a selectable common type for all languages.
Being on the international rules, perhaps a chance to go a step ahead.
Could or should it be taken into consideration?
This thousands separator in the different numbers dialogs should respect the font/cell language as every language has a different default separator recommendation.
If the user doesn't want use this recommended separator for a cell or a style, an 'Edit' button to the right of the thousands separator option should be offered where the user can edit the separator. Every character and special character and other official separator from countries/institutions should be possible to use.
It shouldn't be a property of single number formats, but a general separator and formats settings for the current locale instead, which bug 46448 is about.
I think we can close this one here as a duplicate.