Summarize the following:
You'll find this in your list of all apps, labeled "Translate." You'll find this in the upper-left corner.  Note that not all languages that Google Translate supports are available for offline use. It looks like a down arrow pointing at a horizontal line. You'll be shown the size of the offline translation files before the download starts. Your device will need to have an internet connection in order to download language files for later use offline. The language will be moved to the "Downloaded languages" section, and you'll see a progress circle.

Summary:
Open the Google Translate app. Tap the ☰ button. Tap Offline translation. Scroll down to find the language you want to download. Tap the "Download" button next to a language. Tap Download to confirm. Monitor the download.