«Stores» → «Configuration» → «Advanced» → «Developer»→ «Translate Inline» → «Enable for Storefront» → «Yes»:
Now, you can translate a frontend label by clicking it with the mouse:
The inline translation is a temporary mode: when you have finished the translation you should switch the option back to the «No» value:
Your changes will be preserved:
After switching the modes you should delete the cache.
If you are in the production mode, you should redeploy the static content.
In your are in the developer mode, you should delete the static content.