docs: pdfdocs: Enable language-specific font choice of zh_TW translations

The "TC" variants are supposed to be the choice for traditional
Chinese documents.

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Reviewed-by: Hu Haowen <src.res@email.cn>
Tested-by: Hu Haowen <src.res@email.cn>
Link: https://lore.kernel.org/r/9d8e8fa2-3e7a-3a0a-1a0d-7d797a605ff8@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>