Fix i18n: messages on sphinxmessages.sty_t are not extracted

CHANGES

@@ -16,6 +16,8 @@ Features added
 Bugs fixed
 ----------
 
+* LaTeX: some system labels are not translated
+
 Testing
 --------
 

babel.cfg

@@ -14,6 +14,13 @@ variable_end_string = %>
 block_start_string = <%
 block_end_string = %>
 
+# Extraction from Jinja2 template files
+[jinja2: **/templates/latex/**.sty_t]
+variable_start_string = <%=
+variable_end_string = %>
+block_start_string = <%
+block_end_string = %>
+
 # Extraction from Jinja2 HTML templates
 [jinja2: **/themes/**.html]
 encoding = utf-8