There is a requirement for adding a specific language which is not available in the portal as the portal logon language and to translate all the portal pages to that specific language.
Firstly you need to check whether the language you are looking for is really supported from SAP side or not ?
For that check the the PAM link depending on your portal versions (Here the link is for 7.4 Version) : NW 7.4 Languages
If that specific language is not listed in the PAM document, it confirms that SAP does not support that language,
To enable portal applications to be displayed in different languages, create resource bundles that hold all language or country-specific text strings or other resources, it can be done according to the following help docuement : Using Resource Bundles
Further Portal provides a process for translating the text of portal objects : ‘Portal Content Translation‘