I think your approach is most appropriate. If someone wanted to search and find all performances by a certain artist (Rex Harrison for example), this allows for such discovery.
I think I would have followed the sleeve with the translation on the inner sleeve notes so as Character Name (Vocalist Name). Don't forget that those actors who could not sing in films would have their voices dubbed by others and therefore the reporting as Character Name is strictly correct. Note for this album I am not suggesting any artist was uncreditedly dubbed in whole or part (high note, e.g.?) but make the general point.
Pridesale is right with his suspicion that some actors might have been dubbed for their singing parts. Emma Fairfax was played by Samantha Eggar, but all her songs were sung by Diana Lee, who also performed as an uncredited singer on "Willy Wonka and the Chocolate Factory" (Cheer Up, Charlie!) a few years later (http://www.imdb.com/name/nm0995242/?ref_=ttfc_fc_cl_t26) and on Disney's "Songs From The Electric Company T.V. Show" http://www.45worlds.com/vinyl/album/1350. So I have replaced Samantha Eggar with Diana Lee and added a comment in the Notes.