[FIX] base: avoid confusion between synchronize and update terms

The action "Load a Translation" is wrongfully considered as only for
installation of a new language (while it is for update too).

The action "Synchronize Terms" is wrongfully considered as to refresh
translations of installed language (while it is to generate empty translations).
Rename to "Generate Missing Terms"

The res.lang tree view encourage this misconception with a "update terms" that
actually triggers a synchronize terms action.
Change the triggered action to the "Load & Update"

Move the list of languages to always visible in settings.
This list is not very technical and will avoid to go to debug mode for basic
actions like updating the sources of checking what languages are installed.
Going less often to dev mode will hide the misleading "Synchronize Terms" action
for normal users.

Closes #19911