Translating site, description and copyright for twentyfifteen

Hello, a new user here. In a single-language site, I am trying to translate site, description and copyright text for TwentyFifteen. TwentyFifteen provides a languages.yaml file, which I'd like to customize for my language (Polish) I created a /user/languages/pl.yaml file with the following text:

>     THEME_TwentyFifteen:
>       SITE:
>         TITLE: my_site_title
>         DESCRIPTION: my_description
>         COPYRIGHT: © Copyright %s my_desired_copyright_notice

Yet, the title, description and copyright displayed on a webpage remains a default translation provided in the theme's languages.yaml.

I reviewed the [Translation of site title, description and metatag] (/forum/archive/translation-of-site-title-description-and-metatag-t3409) thread, which also suggests creating a /user/languages/...yaml file.

did you add your language in your site configuration?

Hi Paul, yes, my user/config/system.yaml contains:

languages:
  supported:
    - pl
  include_default_lang: true
  translations: true
  translations_fallback: true
  session_store_active: false
  http_accept_language: false
  override_locale: false

And user/config/site.yaml contains:

default_lang: pl