Flatten threads: localize missing string

The label "Nested view" hadn't been localized, and was instead hardcoded
into the code. This CL adds localization for this string.

Bug: twpowertools:153
Change-Id: I4dc50c0594548f8c77e694ca497f3f5b6c171240
diff --git a/src/lit-locales/source/fr.xlf b/src/lit-locales/source/fr.xlf
index 399d109..ecaf7f5 100644
--- a/src/lit-locales/source/fr.xlf
+++ b/src/lit-locales/source/fr.xlf
@@ -17,6 +17,10 @@
 <trans-unit id="s6b0ed0c74667d28c">
   <source><x id="0" equiv-text="${authorName}"/> said:</source>
 </trans-unit>
+<trans-unit id="s98a0ccd6f423421f">
+  <source>Nested view</source>
+  <note from="lit-localize">Label for the switch which lets users enable/disable the nested view in a thread.</note>
+</trans-unit>
 </body>
 </file>
 </xliff>