Localization of WP theme

Wrap the elements of $search and $replace arrays into __() function to get the translated strings:

function search_replace_search() {
    $search_term = esc_attr( apply_filters( 'the_search_query', get_search_query( false ) ) ); // Get search term

    $search = array(
        __('word1'),
        __('word2')
    );

    $replace = array(
        __('something 1'),
        __('something 2')
    );

    $replacePairs = array_combine($search, $replace);
    echo strtr($search_term, $replacePairs); 
} 

echo search_replace_search();

If you use your own textdomain:

    $search = array(
        __('word1', 'my_textdomain'),
        __('word2', 'my_textdomain')
    );

    $replace = array(
        __('something 1', 'my_textdomain'),
        __('something 2', 'my_textdomain')
    );

More information:

• __() function
• Very good reference on internationalization
• WordPress Internationalization