If you want to make your own portuguese translations you can install the plugin called “codestyling localization”. This will enable you to create a portuguese translation for all of the text in the theme.

But if you only want to change the text on the “Read More” button here are the steps

1. Install the asteroid child theme.
2. Edit the child theme’s functions.php (Dashboard -> Appearance -> Editor)
and put this code:

function asteroid_read_more_button_text($text) {
    return 'New Text';
}
add_filter('asteroid_continue_reading_text','asteroid_read_more_button_text');

Replace the ‘New Text’ with your own words. Then save.