I gave a more advanced version with MultiReplace a go. But try version of the previous post first.

Enable ‘Use Variables’ before adding each line into the list.

By changing the number for LANG in the init part from 1 to 3, you can switch between the language replacements. Just right click on the entry and choose 'Edit Field ’ in the Context menu.

But I have to mention. It is necessary to combine the init command with a single match that appears first in the document; otherwise, the init will not be triggered and the following lines are failing. Or you put the init into each line.

So list look finally like this:

Simply copy-paste this raw data into the replacement list, for testing:

1,"ID0001","init({LANG=3});set((({""Bicycle"", ""Fahrrad"", ""Bicicletta""})[LANG]) );",0,0,1,0,0
1,"ID0002","set(({""Mountain Bike"", ""Mountainbike"", ""Bicicletta da montagna""})[LANG]);",0,0,1,0,0
1,"ID0003","set(({""Airplane"", ""Flugzeug"", ""Aeroplano""})[LANG]);",0,0,1,0,0

To do this, create a replacement list in the MultiReplace Plugin that looks like this:

You can also generate the Replacement list as it is stored in CSV format.

Somebody else might chime in with a way to use Notepad++ builtin functionality or plugins to solve your problem, but IMO that is much less efficient than just using a scripting language.