Translating custom strings into theme knockout templates

You need to extend a language package, creating a translation file in the theme is not enough.

This extension module needs to contain:

composer.json

{
  "name": "snowdog/project-language-translations",
  "description": "Some translations for some project",
  "type": "magento2-language",
  "require": {
    "magento/framework": "^100.0.0"
  },
  "autoload": {
    "files": [
      "registration.php"
    ]
  }
}

language.xml

<?xml version="1.0"?>
<language xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:App/Language/package.xsd">
    <code>en_US</code>
    <vendor>snowdog</vendor>
    <package>en_us_custom</package>
</language>

registration.php

<?php
\Magento\Framework\Component\ComponentRegistrar::register(
    \Magento\Framework\Component\ComponentRegistrar::LANGUAGE,
    'snowdog_en_us_custom',
    __DIR__
);

And of course file with translations in desired language.