Without knowing which version of Elgg you are using I assume that you use some Elgg 2.x or 3.x version. The core language files are in the vendor/elgg/elgg/languages/ subfolder within the install directory of your Elgg site.

But you shouldn't modify the core files directly. It would be just a hassle if you upgrade your site to re-do the changes all the time. You can override the language strings of Elgg core (or any plugin) by creating a plugin that contains a language file for the target language you want to modify a language string of. You would just have to copy the corresponding line from the original language file and insert it in the language file of your plugin and there you can modify the string (right of =>) then. For changing language strings of other plugins you just have to make sure that your plugin is ordered below the other plugins for the change to take effect.

As an example of how such a plugin would have to look like you can use the "Rename the Wire" plugin (https://elgg.org/plugins/1497293). Just drop the language strings you don't want to modify from its language file and add your own (from the Elgg core or other plugin language files) you want to change. You can also add language files for other languages you want to make changes.

I'll try it

Thanks a lot