Check for translated help files in doc directory

After the commit 543041b, :Helptags checks only for the English help files.

I'm using https://github.com/vim-jp/vimdoc-ja which is a project to translate the Vim help files into Japanese, it contains only the Japanese help files (*.jax). Before the commit 543041b, :Helptags generates the help tags but after that it doesn't.