The Swedish number format as used by {{formatnum:}} contains a non breaking space as thousands separator. This is correctly sorted by the table sorter when data-sort-type=number. Without data-sort-type this used to be autodetected as numbers and sorted correctly. But I think something was changed recently. Now without data-sort-type the column is sorted as text, even if it only contains formatted numbers.

Example:
In a Swedish language wiki (e.g. sv.wikipedia.org)

{| class="sortable wikitable"
! data-sort-type="number" | type=number<br> sorts correctly
! no type<br>sorts as text
! no type<br>sorts as number
|-
| {{formatnum:4000}} || {{formatnum:4000}} || {{formatnum:400}}
|-
| {{formatnum:3.9}}  || {{formatnum:3.9}}  || {{formatnum:3.9}}
|-
| {{formatnum:4.12}} || {{formatnum:4.12}} || {{formatnum:4.12}}
|-
| {{formatnum:4.2}}  || {{formatnum:4.2}}  || {{formatnum:4.2}}
|}

1. The first column with data-sort-type sorts as expected.
2. The second column with thousands separators and without data-sort-type is not autodetected as number and sorts as text. Thousands separator and decimal commas are not recognised as numbers. Sorts 4,2 incorrectly before 4,12.
3. The third column without thousands separators and without data-sort-type is autodetected and sorted as numbers. Correctly sorts 4,2 after 4,12.

Expected results would be that all columns with only formatted numbers are autorecognised as numbers.