Override translation of 1 word

Since your problem resolves around the fact that the WP theme developers used a function that doesn’t have any hooks so we could easily alter the output, you’re going to have to copy & paste the following code in your functions.php file

function human_time_diff( $from, $to = '' ) {
    if ( empty( $to ) ) {
        $to = time();
    }

    $diff = (int) abs( $to - $from );

    if ( $diff < HOUR_IN_SECONDS ) {
        $mins = round( $diff / MINUTE_IN_SECONDS );
        if ( $mins <= 1 )
            $mins = 1;
        /* translators: min=minute */
        $since = sprintf( _n( '%s min', '%s mins', $mins ), $mins );
    } elseif ( $diff < DAY_IN_SECONDS && $diff >= HOUR_IN_SECONDS ) {
        $hours = round( $diff / HOUR_IN_SECONDS );
        if ( $hours <= 1 )
            $hours = 1;
        $since = sprintf( _n( '%s hour', '%s hours', $hours ), $hours );
    } elseif ( $diff < WEEK_IN_SECONDS && $diff >= DAY_IN_SECONDS ) {
        $days = round( $diff / DAY_IN_SECONDS );
        if ( $days <= 1 )
            $days = 1;
        $since = sprintf( _n( '%s day', '%s days', $days ), $days );
    } elseif ( $diff < 30 * DAY_IN_SECONDS && $diff >= WEEK_IN_SECONDS ) {
        $weeks = round( $diff / WEEK_IN_SECONDS );
        if ( $weeks <= 1 )
            $weeks = 1;
        $since = sprintf( _n( '%s week', '%s weeks', $weeks ), $weeks );
    } elseif ( $diff < YEAR_IN_SECONDS && $diff >= 30 * DAY_IN_SECONDS ) {
        $months = round( $diff / ( 30 * DAY_IN_SECONDS ) );
        if ( $months <= 1 )
            $months = 1;
        $since = sprintf( _n( '%s month', '%s months', $months ), $months );
    } elseif ( $diff >= YEAR_IN_SECONDS ) {
        $years = round( $diff / YEAR_IN_SECONDS );
        if ( $years <= 1 )
            $years = 1;
        $since = sprintf( _n( '%s year', '%s years', $years ), $years );
    }

    /**
     * Filter the human readable difference between two timestamps.
     *
     * @since 4.0.0
     *
     * @param string $since The difference in human readable text.
     * @param int    $diff  The difference in seconds.
     * @param int    $from  Unix timestamp from which the difference begins.
     * @param int    $to    Unix timestamp to end the time difference.
     */
    return apply_filters( 'human_time_diff', $since, $diff, $from, $to );
}

‘%s min’, ‘%s mins’

‘%s hour’, ‘%s hours’

‘%s day’, ‘%s days’

‘%s week’, ‘%s weeks’

‘%s month’, ‘%s months’

These are the formats you need to change with your own wording; be sure to only change the words (minute, hour, day, week, month) and not the %s

Hope this helps.