You should see a screen like this listing out all the Mifos chapters:
Select a chapter that is untranslated and click translate to begin. You'll see the current text on the left and an empty text editor on the right to input the translated text.
Select all of the English text and images on the left (CTRL+A). Copy and paste this into the right hand text editor window to preserve formatting, headers, links, and images. Once pasted, you can begin translating.
Begin translating! When you're done with the page, change the status from Untranslated to Unpublished and click Save.
What do I do about images?
If you'd like to update the screenshots to be language appropriate, go to the corresponding language-specific Mifos test instance find the relevant section in the UI, and take a screenshot.
Insert the screenshot using the image editor. Images get uploaded in the directory for that translation (i.e. /Mifos_ES for Spanish).