CiviVolunteer - Where to put translations file on WordPress
Based on this https://github.com/cividesk/com.cividesk.l10n.update/blob/master/l10nupdate.php#L131 it looks like it needs to be within the extension folder in folders 10n//LC_Messages/volunteer.mo This line details the download path for the mo https://github.com/cividesk/com.cividesk.l10n.update/blob/master/l10nupdate.php#L130 the mo is the binary version of the .po files i believe