I am using SQuick - Starter eCommerce Template and there is not Language folder for it. I went through the address URL you told me and have not found language folder. My sellacious version looks like 1.7.3
You can manage this key in the ROOT/templates/TEMPLATE_NAME/languages/en-GB/LANGUAGE FILE.
I suggest you use the latest version of sellacious, which has many more features like SMS, chat between sellers/buyers, updated Delivery/Pickup Extension, Hyperlocal, etc. You can check the demo here. https://demo.sellacious.com/squick
As for the language key, go to language/en-GB/en-GB.tpl_squick.ini. Check the folder of the language you are using on your site.
I couldn't find the file en-GB.tpl_squick.ini. and have searched in few different folders. I have tried to update the system manually by following the instructions and upload the files but it did not work well and my website went down. I ended up restoring the database and website files to bring my website back to live. Also, I tried to update via Joomla but had the following error as you can see on the attached image.
Could you help me by any chance by having a look at my website? I meant for the missing language string and possibly updating?
You can download the 2.0 version from our site or update it from Joomla. You can check the documentation about the update. Make sure to take a backup of your site. However, if you update the sellacious, you won't have an issue with the language file.