Urdu language not listed in my ubuntu 16.04 LTS

Language Support includes the languages for which language packs have been created, and language packs are no longer created for languages with a translation coverage below a certain threshold (can't tell exactly what the limit is). Probably that's the explanation why you don't find Urdu.

You may want to install the fonts-nafees package to render Urdu web contents.

sudo apt install fonts-nafees

Possibly you may also want to contact the Urdu translators team and help improve the translation status, so the Urdu language packs can again be included in the distribution. (They were included in Ubuntu 12.04 and 14.04.)