Ise,
You can change this into Portuguese in the Language customisation area.
In Site administration > Language > Languages customisation, open the Portuguese language pack for editing. In the "Show strings of these components" list, select "h5pactivity.php", and for the string identifier, enter in "previewmode" and click the Show strings button. You will then see box with an area for local customisation. Enter your translation into the box and save this. Please see the attached screenshot, which is an example of how this looks in the Spanish language pack, which already has a translation for this string.
Long term, you should also contact the maintainer of the Portuguese pack with your translation and ask them to add it to the pack so everyone will get the update. You can find out about that at
AMOS,
the Moodle translation site.
