To import a language of PrestaShop you must make sure that these folders have chmod 777. To do that just follow the steps:

• Check and set permissions to folders:

/mails
/modules (here need to check each directory, if there are subdirectory named [/mail] then you need to set chmod 777 for them too)
/themes/prestashop/lang  (if you use a custom theme then you must manually import the your_language.php file)
/translations
/upload

• If you plan to update manually the you need to make sure that the host supports *.gzip

Most important of all is to make sure that all subdirectories named [mail] in [/modules] folder are all in chmod 777. This is the main reason for unsuccessful import beacause lack of redundancy in applying chmod 777 on most if not all hosting servers.

In addion you can edit your language file under  Tools > Translations > Modify Translations >  [click flag to select your language]. To do so you need to change chmod 777 for [/themes/your_theme/lang/your_language.php]