Role/Capability: Prevent a fatal error when checking a language capability...

Role/Capability: Prevent a fatal error when checking a language capability early or in the frontend.

The meta capabilities `install_languages` and `update_languages` rely on the admin function `wp_can_install_language_pack()`. That function in turn relies on `request_filesystem_credentials()`, so it needs to be ensured that this function is available as well.

Fixes #42230.

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git-svn-id: https://core.svn.wordpress.org/trunk@41719 1a063a9b-81f0-0310-95a4-ce76da25c4cd