"English only..." - For traditional add-ons, developers could use the free BabelZilla web-based translation management system to coordinate submissions from volunteer translators. At present, for WebExtensions add-ons, there is no equivalent web-based translation management system.
"Only 3 items..." - The toolbar button right-click menu can only allows a maximum of 6 add-on menu items. If you want to see 6 URL levels on this menu, you can disable the 'Navigate Up One Level' and 'Navigate Up To Root' menu items. Note, the 'Navigate Up WE' submenu on the normal context menu shows all available URL levels.
"Why to move out of address bar..." - For compatibility with the Chrome version and because several users requested this change. It would be possible to have an option to place the button in the address bar instead of on the toolbar.