First, that isn't a "plugin". It is an extension, which is a different type of add-on. Firefox supports very few plugins these days, like Flash is the only plugin which is now supported.

https://addons.mozilla.org/en-US/firefox/addon/s3google-translator/ Right-click the + Add to Firefox button and select Save Link As ... using a computer that can connect to the internet and save that XPI file.

Then transfer the s3translator-6.15-an+fx.xpi file that you saved to the other PC using a USB Flash stick. On that PC drag that XPI file into the Firefox browser window and Firefox will install it, if that XPI is compatible with the version of Firefox that you have installed there. If not compatible Firefox will state that incompatibility.

The real problem isn't about installing the extension, but about whether you can make the extension work.

I don't think that any of the translate extensions will work in offline mode because they send content to a web server via XHR to do the translation and this obviously isn't possible in offline mode. You would need a builtin dictionary and parser to be able to do this offline.