Custom translation file (Not using Locales/Multisite) Is it possible to make Craft 3 load a custom translation/language file? Even if not using multisite or locales?

My use case for this is just to change the way certain things are said. E.g. 'Invalid username or password.' =&gt; 'Invalid username or password.', when a user logs in. I wanted to be a bit more descriptive on this.

I can see the translation files in vendor/craftcms/cms/src/translations/[language code here] so I assume theres a way of adding bespoke ones to this, but outside of the vendor folder. 

I just can't find it in the docs.
You are looking for Static Translations.

You can add a translation file for English to English (or whatever language you are using) to override default translations. In your case, you'll want to override the app translation category:

In your project folder:

website.com/
└── translations/
    └── en/
        └── app.php


And in app.php:

&lt;?php

return [
    'Invalid username or password.' =&gt; 'Something new'
];