Add a comment for translators to clarify "Pixel aspect ratio"

Also update the French translation

data/ui/clipmediaprops.ui

@@ -193,7 +193,7 @@
                             <property name="spacing">6</property>
                             <child>
                               <object class="GtkCheckButton" id="PAR_checkbutton">
-                                <property name="label" translatable="yes">Pixel aspect ratio:</property>
+                                <property name="label" translatable="yes" comments="PAR is the aspect ratio &quot;of the pixels&quot;, not an aspect ratio &quot;in pixels&quot;.">Pixel aspect ratio:</property>
                                 <property name="visible">True</property>
                                 <property name="can_focus">True</property>
                                 <property name="receives_default">False</property>

data/ui/projectsettings.ui

@@ -326,7 +326,7 @@
                     </child>
                     <child>
                       <object class="GtkRadioButton" id="select_dar_radiobutton">
-                        <property name="label" translatable="yes">Display aspect ratio</property>
+                        <property name="label" translatable="yes" comments="DAR is the aspect ratio of the image being displayed to the viewer. &quot;Display&quot; is used as a noun, not as a verb/action.">Display aspect ratio</property>
                         <property name="visible">True</property>
                         <property name="can_focus">True</property>
                         <property name="receives_default">False</property>
@@ -381,7 +381,7 @@
                     </child>
                     <child>
                       <object class="GtkRadioButton" id="select_par_radiobutton">
-                        <property name="label" translatable="yes">Pixel aspect ratio</property>
+                        <property name="label" translatable="yes" comments="PAR is the aspect ratio &quot;of the pixels&quot;, not an aspect ratio &quot;in pixels&quot;.">Pixel aspect ratio</property>
                         <property name="visible">True</property>
                         <property name="can_focus">True</property>
                         <property name="receives_default">False</property>

po/fr.po

@@ -343,13 +343,15 @@ msgstr "Enregistrer les modifications du pré-réglage actuellement sélectionn
 msgid "Aspect Ratio:"
 msgstr "Rapport d'affichage :"
 
-#: ../data/ui/projectsettings.ui.h:11
+#. DAR is the aspect ratio of the image being displayed to the viewer. "Display" is used as a noun, not as a verb/action.
+#: ../data/ui/projectsettings.ui.h:12
 msgid "Display aspect ratio"
 msgstr "Rapport d'affichage à l'écran"
 
-#: ../data/ui/projectsettings.ui.h:12
+#. PAR is the aspect ratio "of the pixels", not an aspect ratio "in pixels".
+#: ../data/ui/projectsettings.ui.h:14
 msgid "Pixel aspect ratio"
-msgstr "Rapport d'affichage en pixels"
+msgstr "Rapport d'affichage des pixels"
 
 #: ../data/ui/projectsettings.ui.h:13
 msgid "x"
@@ -1601,7 +1603,11 @@ msgstr "Image PNG"
 msgid "JPEG image"
 msgstr "Image JPEG"
 
-#: ../pitivi/dialogs/clipmediaprops.py:82
+#: ../pitivi/dialogs/clipmediaprops.py:101
+msgid "invalid (%s fps)"
+msgstr "invalide (%s fps)"
+
+#: ../pitivi/dialogs/clipmediaprops.py:123
 msgid "Image:"
 msgstr "Image :"