In the PO file, there mentioned a piece of text, a tooltip for 'Area Weight':
#. :src: bpy.types.UV_OT_smart_project.user_area_weight
msgid "Weight projections vector by faces with larger areas"
Checking this against the original manual on this page:
it turned out that the text should be:
Weight projection's vector by faces with larger areas
In the word 'projections', the 's' should indicate 'possesion', instead of plural as typed. It needs an apostrophe before the 's'.