Standard text of "MediaWiki:Tpt-saveok" (displayed after successfully marking for translation) in mediawiki.org was updated by @Nemo_bis with addition folowing phrase:
Please import any pre-existing translations: you can use [[Special:PageMigration]] for this purpose.
May be useful, if this text will be included into core functionality
- as addition to "MediaWiki:Tpt-saveok"
- or as separate message like "MediaWiki:Tpt-saveok-firstly", will be displayed only after first marking for translation.
Second option look more preferable.