About the theme (the look).
Getting the old look back will be very difficult, since that theme is not working in Plone 5 (basically none of the old themes work, since the whole 'theming stuff' is different..
That said: go to /@@theming-controlpanel and Choose 'Bareceloneta theme' , then click on the "Advanced Tab" and choose 'No styles (or something similar) as Base Theme.
You will now have the new Plone 5 theme, that probably is OK.
You can change the logo in the 'Site' control panel.
About the other error I only have a wild guess: you have enabled several languages on your site and used an add-on that is not working in Plone 5, you could try to uninstall it and then run the upgrade
( http://yoursite/manage_main and click on 'portal_quickinstaller'
Espen