Global language files vs plugin files

It’s not clear from your questions whether you intend to have more than one file of the same language. Are you creating custom versions of a language that’s already provided by the author?

There is a default language file inside the plugin’s language folder.

I’m unclear what you mean by a “default language file”. If you see something like default.po then the plugin is set up badly and you will probably run into further problems. If you want to know for sure, please post a link to download your plugin.

I would like to change some strings and thus created a custom file in the global languages folder

This is done by adding a new language in Loco Translate. Have you tried this yet? If it can understand your plugin it will create a localised version of the “default” file in the language you choose. It won’t overwrite any existing files.

What will happen now?

When WordPress looks for translations in your language it will first look wherever your plugin has asked it to. If it doesn’t find a matching file in your language it will only then look in the global languages directory.

Will the plugin first use the global file and then fall back on the default plugin file

I don’t know what your plugin will do, but my plugin isn’t involved in this process.

I’m glad this worked, but a better solution would be to use the override_load_textdomain filter if you’re able to.

You’ll have to write your own code for this. If you’re not sure, stick to your current methods.