Testing in 1.5.0.1
(This list was laid down for Magento developers, but it also could be useful for those who use translation)
(This list is far from complete...)

1. Original (en_US) Mage_Sales.csv lacks following phrases:
* There are no recurring profiles yet.


2. Original (en_US) Mage_Catalog.csv lacks following phrases:

* Please specify the product's option(s).
* Display Settings


3. In app\design\adminhtml\default\default\template\tax\importExport.phtml (45, 61) some phrases don't use translation at all. You can fix this by replacing

45: from <?php echo $this->getButtonHtml('Import Tax Rates', "this.form.submit()") ?>
    to   <?php echo $this->getButtonHtml(Mage::helper('tax')->__('Import Tax Rates'), "this.form.submit()") ?>

61: from <?php echo $this->getButtonHtml('Export Tax Rates', "this.form.submit()") ?>
    to   <?php echo $this->getButtonHtml(Mage::helper('tax')->__('Export Tax Rates'), "this.form.submit()") ?>

4. To enable translations for Index Management Page insert following code in /app/code/core/Mage/Index/etc/config.xml after line 143

<translate>
	<modules>
		<Mage_Index>
			<files>
				<default>Mage_Index.csv</default>
			</files>
		</Mage_Index>
	</modules>
</translate>

5. There is no possibility to translate category attributes

6. There is no possibility to translate customer attributes

7. Original (en_US) Mage_Adminhtml.csv lacks phrases:
* "Last updated: %s. To refresh last day's <a href=""%s"">statistics</a>, click <a href=""%s"">here</a>."

8. To enable translations for Magento Connect - Package Extensions page add following code to /app/code/core/Mage/Connect/etc/config.xml after line 64

<translate>
	<modules>
		<Mage_Connect>
			<files>
				<default>Mage_Connect.csv</default>
			</files>
		</Mage_Connect>
	</modules>
</translate>

9. To enable translations of review statuses in list of reviews change line 77 in app\code\core\Mage\Adminhtml\Block\Review\Grid.php

	$tmpArr[$status['value']] = $status['label'];
to
	$tmpArr[$status['value']] = $this->__($status['label']);

that will make Magento to take statuses from Mage_Customer.csv

10. Statuses of price rules have no translations. You can see that code doesn't provide for such possibility
	app\code\core\Mage\Adminhtml\Block\Promo\Catalog\Grid.php (93)

11. There is no translation for editor because of some code peculiarities. Like, there are possibilities for translation, but there is a check, if object which manages translations is extended from Varien_Object. In this case such object is Mage_Core_Helper_Abstract which does not extend Varien_Object and translations are not taken.

	lib\Varien\Data\Form\Element\Editor.php (line 345)
	if ($this->getConfig('translator') instanceof Varien_Object)

Theoretically you can just comment this check out, but we don't know if it affects something else. However we had no problems for now and translation works fine.

12. No translation for phrase "Display On"
app\design\adminhtml\default\default\template\widget\instance\edit\layout.phtml (43)

13. No translations for Import, Export, Product, Customer on the export/import profiles list
	app\code\core\Mage\Adminhtml\Block\System\Convert\Gui\Grid.php (69, 76)

14. Run Profile tab: no translation for "select a file"
	app\design\adminhtml\default\default\template\system\convert\profile\run.phtml (49)

15. No translations for stability statuses
	app\code\core\Mage\Adminhtml\Model\Extension.php (328)


16. No translations for roles
	app\code\core\Mage\Adminhtml\Model\Extension.php (251)

============ reported ================
============= new =================

17. Original (en_US) Mage_Adminhtml.csv misses the following phrases
* "Alphanumeric, dash and underscore characters are recommended for filenames. Improper characters are replaced with '_'."

There is a phrase "Alphanumeric, dash and underscore characters are recommended for filenames. Improper characters are replaced with \'_\'.", but it is'n being used

18. There is no possibility to translate parameter group comments in configuration in admin.

To solve this problem you should change app\code\core\Mage\Adminhtml\Block\System\Config\Form.php (183):
Replace the following string
$fieldsetConfig['comment'] = (string)$group->comment;
with
$fieldsetConfig['comment'] = Mage::helper($helperName)->__((string)$group->comment);

It is used in System - Configuration - Inventory - Product Stock Options