How the language package works

Summary:

Hello,
we are opening offices in Quebec and would like to understand how the language package works.

We have read that the language package must be applied to all environments at the same time but can we disable it in production while we test and verify the functionality in the non-production environments?

Also:

1) how is this enabled by users? In "Set preferences"?

2) does the package translate custom values? For example in data entry, we have created values like, "front line worker", "JDE contingent workers", etc.. to categorize our workforce? Does the language package translate all these values, or do we need to update the values to include the French translation?