Add trying to access README according to LANG

[ono-taka179]

This is one of the proposed implementations for handling access to the translated README.
It is assumed that the file is located on the server under the name README-{LANG}.pdf (e.g. README-ja_JP.pdf).

[pep8speaks]

Hello @ono-taka179! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file usr/lib/python3/dist-packages/drauger_welcome/main_ui.py:

Line 381:80: E501 line too long (80 > 79 characters)
Line 389:9: E722 do not use bare 'except'
Line 391:80: E501 line too long (91 > 79 characters)

[ono-taka179]

Attempts to access the translated README and opens the original README if an exception occurs.
To avoid 403 access to the server, a custom User-Agent is specified in the header.