When using number custom fields it will always show in US format. If the customer set Trello to be used in another language the custom field won't reflect the customer local number format.
For example, in Portuguese, comma (,) is used as the decimal separator and period (.) as the thousands separator.
This differs from English formatting (e.g., 100,000 for US format and 100.000 for BR).

User would like to have the number custom field to reflect their local number formatting when setting the language in Trello to their idiom.

This way, when exporting the board as a CSV file, the number custom field would match the formatting of their local one.