Use locale defined digits instead of 0-9

[snoopyjc]

I would like to use babel to convert numbers into locale-defined characters. For example in zh_TW, 1350 is represented as 一千三百五十. Your format_decimal(1350, locale='zh_TW') returns 1350.

[akx]

This was somewhat implemented in #470 (which would need rebasing), but we'd also need to be very careful with backward compatibility; a Babel version upgrade can't have numbers start getting spelled differently...

Also ref #446

[snoopyjc]

Thanks - it's not a 1-to-1 mapping though. For example, several languages (e.g. Chinese, which is also used in Japanese) have digits for 10, 100, 1000, etc, so you write 1350 as One Thousand (一千) Three Hundred (三百) Five Tens (五十). Ref: https://en.wikibooks.org/wiki/Chinese_(Mandarin)/Numbers

[snoopyjc]

@akx maybe follow Excel’s lead where they add [DBNumN] to the number format to use locale digits, ie add an extra parameter to format_decimal to enable it.