Updated: Comment #0
1.) When adding a new language on admin/config/regional/language, the language files are not downloaded.
The site is installed in English and French language is added:
sites/default/files/translation is empty.
sites/default/files/languages doesn't exist.
When installing the site in French, on admin/config/regional/language it is indicated that 77.43% of the strings are translated.
The same happens with other languages.
2.) When installing the site in another language than in English, the downloaded .po files are those for Drupal 7:
On an installation in French:
sites/default/files/translation > drupal-7.0.fr.po
sites/default/files/languages > fr_V1w2Fpm_cFVONuoPeAW5VNt_uVVj_2p-Lr7fiyzv_6w.js
On an installation in Finnish:
sites/default/files/translation > drupal-7.0.fi.po
sites/default/files/languages > fi_uzcPQ6tyBuW6lg6wdOrZKWz_V1d8uL_1p5Hg_1D1f9I.js
User interface changes
At the moment, when adding a new language, all the strings on admin/config/regional/translate/translate are empty:
I suppose that if the language files were downloaded, some of them would be translated as they are when the site is installed in French: