Takes under a minute. You add Inbox Translate once and it appears in every Outlook you sign in to with the same account.
You'll need this add-in URL:
https://app.inboxtranslate.com/manifest.xml
Open any email and click Inbox Translate in the ribbon (or under Apps). Sign in with your Microsoft account the first time. When you're writing a reply, use Reply in language. The first translation downloads the on-device model (about 40 seconds, only once). After that it works offline and your email never leaves your computer.
A Microsoft 365 admin can push it to everyone at once with no action needed from users. See Deploy to your organization.
If Custom add-ins is missing, your IT department may have disabled it. Ask them to deploy it for you. Otherwise email support@inboxtranslate.com.