The Translate plugin is a 32-bit-only plugin that hasn’t been updated since 2015. If you have the 32-bit Notepad++, it will show up in your Plugins Admin; since you say you don’t see it, I am assuming you have the 64-bit Notepad++ instead.

But since the plugin hasn’t been updated since 2015, even if you were to switch to 32-bit Notepad++ (which I don’t think is really a good idea), it is highly likely that the plugin would no longer be compatible, because Notepad++'s plugin interface and underlying interactions have changed significantly in the last eight years, and the chances are that some or all of the features as implemented are not compatible with modern Notepad++.