If you changed something in child theme (we mean you modified files or added ow functions) then of course you will lose those changes. But child theme can be updated the same way as parent theme.
What Language Course you mean? Please explain what exactly does not work because we do not see anything at all on our side. For us everything looks fine. Only this background in the top does not look good. This is header background section and if you don't need it, you can remove it by activating "Header | Minimalist" option under Theme options > Header & Subheader > Header section.
Comments
what is not smooth exactly? We did checked your site and do not see anything wrong.