方法 1)
Go to Settings / Installed Modules
In search field enter "base" as a filter
Open "base" module in form view
Click on upgrade
This should load and update all .po files, because all addons have base as minimum requirement (dependency).
方法 2)
Ok, I found that all translations are stored to database in 'ir_translation' table. While it would be convenient to somehow force the translation table to update from changed .po files, I resolved just to edit the necessary missing translations directly in database.
Searching for specific translation entry is actually faster using database search, than hunting it from the addons subdirectories and .po files.