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/es.xlf b/src/lit-locales/source/es.xlf
index 6161009..3ff72c1 100644
--- a/src/lit-locales/source/es.xlf
+++ b/src/lit-locales/source/es.xlf
@@ -21,6 +21,11 @@
   <source><x id="0" equiv-text="${authorName}"/> said:</source>
         <target><x id="0" equiv-text="${authorName}"/> dijo:</target>
       </trans-unit>
+<trans-unit id="s98a0ccd6f423421f">
+  <source>Nested view</source>
+  <target>Vista anidada</target>
+  <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>