How to translate Osmosis's strings by using the .po/.mo files?

Option 1: From Theme .po .mo Files by using a program like Poedit and named them like en_Us.po (in your language).
- Change text from Osmosis Theme .po .mo files ( Path: \osmosis\languages )

Option 2: From Child Theme .po .mo Files (with the same way like Option 1)
- Upload and Activate Osmosis Child Theme
- Change text from Child Theme .po .mo files ( Path: \osmosis-child\languages )

With Option 1 you will have to update again the .po, .mo files in case you update the Theme to a newer version.
With Option 2 you will not lose your changes even if you upload an updated Theme.

Note: Finally, you need to go to Settings > General and at the bottom of this page select your preferred language. See the screenshot below.