Although translated in Pootle, the Extension Manager remains partly unlocalized if LibO UI language is set to certain languages, including my native Estonian. The buttons "Help" and "Close" are translated in all cases, as are the names and descriptions of the extensions themselves, but for some languages the rest is shown in English.
This has been this way for as long as I can remember in 4.0 branch -- I attributed it to build being out of sync with Pootle, but apparently this is not the case, as the partial localization is still present in 4.0.0 RC 1, which has been synced AFAIK. Tried with a blank user profile on Linux x86 (Debian) and Windows XP.
Tested with some other languages as well, here are the results:
affected by this bug: Estonian, Italian, Esperanto, Spanish, Hungarian, Basque, Japanese (all have the strings translated in Pootle) + possibly more;
unaffected: French, German, Dutch, Finnish, Swedish, Norwegian Bokmål, Danish, Scottish Gaelic, Catalan, Portuguese, Romanian, Greek, Russian + probably more.
Slovenian translation strings seem to show properly.
Apparently this is somehow caused by Pootle, see http://listarchives.libreoffice.org/global/l10n/msg05397.html
Looks like this is not our bug -- resolving as such.