All texts that can be seen by respondents should appear in the file when exporting a study for translation. I did a quick test with Lighthouse Studio 9.6.1 and found the next button and previous buttons texts to be the first two items listed. Is this different than what you see when you export your questionnaire for translation?
Of course, all survey texts can be changed through Lighthouse Studio directly, if a text was missed during translation. Specifically, the text for the next and previous buttons can be found in your survey settings, in the "General Settings" tab. Look for "Navigation Buttons."