English and French locales must be installed

Clarifies our Contributing docs to ensure these two locales are present,
since they are required when updating functional test output.

docs/contribute.rst

@@ -76,7 +76,8 @@ mentioning that "some generated files differ from the expected functional tests
 output." If you have made changes that affect the HTML output generated by
 Pelican, and the changes to that output are expected and deemed correct given
 the nature of your changes, then you should update the output used by the
-functional tests. To do so, you can use the following two commands::
+functional tests. To do so, **make sure you have both ``en_EN.utf8`` and
+``fr_FR.utf8`` locales installed**, and then run the following two commands::
 
     $ LC_ALL=en_US.utf8 pelican -o pelican/tests/output/custom/ \
         -s samples/pelican.conf.py samples/content/