Fix #1918153 [[Enhancement] Remember language when going to the calibre homepage from the User Manual](https://bugs.launchpad.net/calibre/+bug/1918153)

manual/conf.py

@@ -93,7 +93,8 @@ unused_docs = ['global', 'cli/global']
 
 locale_dirs = ['locale/']
 title = '%s User Manual' % __appname__
-if language not in {'en', 'eng'}:
+needs_localization = language not in {'en', 'eng'}
+if needs_localization:
     import gettext
     try:
         t = gettext.translation('simple_index', locale_dirs[0], [language])
@@ -174,6 +175,7 @@ def sort_languages(x):
 html_context['other_languages'].sort(key=sort_languages)
 html_context['support_text'] = _('Support calibre')
 html_context['support_tooltip'] = _('Contribute to support calibre development')
+html_context['homepage_url'] = 'http://calibre-ebook.com' + (f'/{language}' if needs_localization else '')
 del sort_languages, get_language
 
 epub_author      = u'Kovid Goyal'

manual/templates/about.html

@@ -7,7 +7,7 @@ function toggle_languages_menu() {
 </script>
 
 <div style="text-align: center">
-    <a href="https://calibre-ebook.com" title="calibre homepage" id="calibre-homepage-logo"><img src="{{ pathto('_static/logo.png', 1) }}" /></a>
+    <a href="{{ homepage_url }}" title="calibre homepage" id="calibre-homepage-logo"><img src="{{ pathto('_static/logo.png', 1) }}" /></a>
     <br/>
     <form id="donate_form" action="https://www.paypal.com/cgi-bin/webscr" method="post" title="{{ support_tooltip }}">
 <style>