Cleanup optional imports

Disable the multiprocessing module on Windows
Update translations

lector/parsers/comicbooks.py

@@ -54,7 +54,7 @@ class ParseCOMIC:
         return title
 
     def get_author(self):
-        return None
+        return 'Unknown'
 
     def get_year(self):
         creation_time = time.ctime(os.path.getctime(self.filename))

lector/parsers/pdf.py

@@ -22,12 +22,7 @@ import os
 from PyQt5 import QtCore
 from bs4 import BeautifulSoup
 
-proceed = True
-try:
-    import popplerqt5
-except ImportError:
-    print('python-poppler-qt5 is not installed. Pdf files will not work.')
-    proceed = False
+import popplerqt5
 
 
 class ParsePDF:
@@ -37,9 +32,6 @@ class ParsePDF:
         self.metadata = None
 
     def read_book(self):
-        if not proceed:
-            return
-
         self.book = popplerqt5.Poppler.Document.load(self.filename)
         if not self.book:
             return