Add option to force default language for public users #944

After adding APP_LANG to the .env file, only the language for logged in user changes and not for the whole site.

Steps to reproduce the behavior:

1. Add APP_LANG=pl to the .env file
2. Log in as admin@admin.com
3. Set Allow public viewing? to yes
4. Log out
5. Homepage is still in english

Expected behavior
Whole site should change language to the polish/french/german etc..

Your Configuration (please complete the following information):

• Exact BookStack Version (Found in settings): 0.23
• PHP Version: 7.2
• Using Docker or reverse proxy (Yes/No): No