The tests currently fail on WP trunk with the following error:

ls: cannot access /tmp/wp-cli-test-suite-cache-language-core.feature.4-5cb78382e69bd8.76370729/translation: No such file or directory

It seems to have for core language, plugin languages and theme languages.

Here's one example of where that is happening: https://travis-ci.org/wp-cli/language-command/jobs/521353616#L720

Related wp-cli/wp-cli#5192