To change magento store language please do following steps:
- install original translation from magento connect. All available translations can be found here http://www.magentocommerce.com/translations
- create app/design/frontend/default/shopper/locale/[your_locale]/
- copy app/design/frontend/default/shopper/locale/en_US/translate.csv to new locale folder and translate it
- switch store locale to your new language in System--> Configuration --> General
- refresh cache ( System --> Cache management )
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