Hi,
I don't think it's linked to the upgrade or update.
If you somewhat lost all your translations, it indicates that there is an error in the translation override file (supposing that you're not using Falang for your translations).
So the first thing I would recommend is to edit your language in the Languages tab of the HikaShop configuration and check the override area if you see your translations. If you see them there, then it's likely that there is an error in the file somewhere and thus HikaShop's translation system cannot read it anymore.
Turn on the "debug language" setting of the Joomla configuration and access a page of HikaShop in your backend. You'll then see the list of the translation files loaded (as well as the override file) at the bottom of the page. And if there are some problems with them, it will tell you which line of which file has a problem. So you can easily find where is the issue.
Then, please report to us your findings. It might be something we didn't expect that we need to support.
We do our best to test everything, but it is a big piece and sometimes, you fix something somewhere, and it breaks something else elsewhere that you did not expect. Also, as it interacts with many things, plugins, other extensions, templates, libraries, PHP versions, Joomla versions, etc you need to make everything you do work with all of this and all the different combinations. It's not something where you can possibly test everything.
When you develop a feature for a client, you know the environment, you only have one set of extensions, one version of PHP, one version of Joomla, etc. And you can directly test your new feature on their environment (or a copy of it) before you introduce your client to it. So it's a whole lot different than software development to tens of thousands of different environments.
I hope you can understand the scale of what we're trying to do. Even with all the money Microsoft (or any other big software company) has, there are bugs in pretty much any software update they do, some because of hardware conflicts, or other software they didn't think of, or new features having issues with other parts.