Why does my translate.csv and the translation files not work?

In some instances, Magento has multiple modules trying to translate the same item. Take Add to Cart for example. In Magento the modules Mage_Catalog, Mage_Checkout, Mage_Reports, Mage_Sales, Mage_Tag, Mage_Wishlist and Mage_XmlConnect all try to translate this string.

What helps in those cases is to specify which translation of which module you are trying to overrule. You can do this by adding a double colon, like so:

"Mage_Catalog::Add to Cart","Bestellen"
"Mage_Catalog::Add to Wishlist","Toevoegen aan favorieten"

There could be any reason below, or a combination of many. So I write down all of them:

1. There must be a store view for German language (System > Manage Stores);
2. Go to System > Configuration > General / General > Locale Options. Choose German language view from Current Configuration Scope (upper left corner). Make sure that German locale is chose;
3. Make sure that the string you want to translate is EXACTLY as it is in translate.csv. Magento is infamous for small tweaks in base texts -- punctuations, capitalizations and so on;
4. Make sure that web server has enough privileges to read translate.csv file;
5. Make sure that there's no precedence taking inline translation in place. Check core_translate table;

Make sure you add your theme in the CMS:

system > config > design > translations <theme-name>

Otherwise translate.csv won't load.