The directory /ext in a language pack is not mentioned in the installation instructions

[url=https://www.phpbb.com/support/docs/en/3.3/kb/article/how-to-install-a-language-pack/]How to Install a Language Pack[/url]

Now that you have the compressed language pack on your computer, you will need to extract (unzip) it. The extracted archive should contain 2 folders:

/language
/styles