Use of multiple languages - Only frontend language seems to be picked up by search enigines

I am using OpenCart version 2.0.3.1 for my webshop. I am not a developer or otherwise familiar with the code of OpenCart, so therefore I would like to put my problem forward in this Forum to see if anyone can help.
3 languages are installed for my webshop. The content, the keywords. -titles and -descriptions (backend) are all applied in these 3 languages. When I went live, I noticed (after some time) that my shop could only be found using terms from the content or the keywords in the language that was chosen as the frontend language. None of the keywords -even combined with the name of my webshop- in the 2 other languages could be found. They seemed to be totally blocked. As soon as I changed the frontend language, my webshop could be found in this new language.
What is the problem? I thought that the possibility of multi-language use (fully applied in the backend) would automatically mean that search engines would pick up content and keywords etc.. in all languages.
Or is it necessary to have 3 sitemaps in 3 languages, in order to make it work? How can I get rid of this blockage?
Thanks in advance.

In OpenCart default SEO is indeed pretty weak. If you want to have full multilingual support and correct sitemap per each language, you need to install 3rd party extensions.

For sitemap I'd recommend you Smart Sitemap. This is probably the most flexible and precise sitemap for OpenCart.

For SEO related features you may use Smart SEO Pack. It'll make your site 100% multilingual, including hreflangs, which are responsible for indexing displaying site in correct language.

Also you might need to adjust your .htaccess file, but that's another story :-)

Thanks for your reply.
But why would I need an additional package to be able to use keywords in multiple languages? The default version is already equipped with this possibility. I have entered all the keywords, descriptions etc. in 3 languages.