Validate translations during build

Incorrect format strings can cause crashes, so we want to catch those
early.

po/CMakeLists.txt

@@ -41,7 +41,7 @@ foreach(lang ${po_FILES})
 
   # Add command to build X.mo from X.po
   add_custom_command(OUTPUT ${mo}
-    COMMAND ${GETTEXT_MSGFMT_EXECUTABLE} -o ${mo} ${po}
+    COMMAND ${GETTEXT_MSGFMT_EXECUTABLE} --check -o ${mo} ${po}
     DEPENDS ${po}
   )