How to add CSS and JS files in magento header?

Open your page.xml file and add the below lines

skin_csscss/YOURCSSFILENAME.css
skin_jsjs/YOURJSFILENAME.js

Refresh your cache. Thats it. You are done.

How to Show Language Switcher in Magento Header or Footer?

Step 1: Create a new phtml file "template/page/switch/stores-top.phtml" and write the following code in it:

<?php if(count($this->getGroups())>1): ?>
<div class="language-switcher" style="margin-left:15px">
    <label for="<span class="store" id="switcher">select</span>-store"><?php echo $this->__('Select Store') ?>: </label>
    <select id="select-store" onchange="location.href=this.value">
    <?php /*foreach ($this->getStores() as $_store): ?>
        <span class="IL_AD" id="IL_AD1">option value</span>="<?php echo $_store->getUrl('') ?>"<?php if($_store->getId()==$this->getCurrentStoreId()): ?> <span class="IL_AD" id="IL_AD4">selected</span>="selected"<?php endif; ?>><?php echo $_store->getName() ?></option>
    <?php endforeach;*/ ?>
    <?php foreach ($this->getGroups() as $_group): ?>
        <?php $_selected = ($_group->getId()==$this->getCurrentGroupId()) ? 'selected="selected"' : '' ?>
        <option value="<?php echo $_group->getHomeUrl() ?>" <?php echo $_selected ?>><?php echo $this->htmlEscape($_group->getName()) ?></option>
    <?php endforeach; ?>
    </select>
</div>
<?php endif; ?>

Step 2: Add store_switcher_top block after store_language block inside header block of "page.xml" present around line number 66.

step 3: Add getChildHtml('store_switcher_top') below getChildHtml('store_language') in "template/page/html/header.phtml" like below.

$this->getChildHtml('store_language');
$this->getChildHtml('store_switcher_top');

Configurable products options are empty after moving my cart side bar to header?

I have explained Moving Cart from Right Sidebar to Header in this tutorial. After moving my cart to header I have noticed configurable options are not shown up in header. To display configurable product options do the following.

Edit your "template/checkout/cart/sidebar/default.phtml" file.
Find the line if ($_options = $this->getOptionList()):

Replace with the following

$_product = Mage::getModel('catalog/product')->load($this->getProduct()->getId());
$config = new Mage_Catalog_Helper_Product_Configuration();  
if ($_options = $config->getConfigurableOptions($_item)): 

Thats it. You are done.