First of all...

Check if that extension is already translated in your language at our forum
section in that extension. A lot community members share their work with
other users speaking the same language and make the translations they
created available for you.

All translations are installable via the Joomla extension installer (You
need Joomla 1.5.x installed). Most of the packages contain an administration
and site application translation.

Note: This requires that within Joomla the choosen language is already
installed. Otherwise the installation will fail with the notice that no
matching Joomla Core translation was installed.

If you can't use the installer, another possibility is to copy the
translation ini files into the matching /languages/{langTag} directory of
your Joomla. Keep in mind that Joomla 1.5 uses separate language files for
the site and administrator application.

* /administrator/language/{langTag} for the Backend
* /language/{langTag} for the Frontend

If there is not that translation, what you need, for your language, you need
to create one from scratch. Ensure you have already installed the Joomla
Core translation of your language. Then download the that Alphaplug.com
extension installation package and extract it. You will notice a
"languages-admin" and "languages-front" folders. In each of this folders is
an en-GB.com_"some_alphaplug_extension".ini file is located. Rename them
that they match with your language tag. For a German translation that would
be in backend like "de-DE.com_alphacontent.ini",
"de-DE.com_alphacontent.menu.ini" and in frontend
"de-DE.com_alphacontent.ini" for example. Now you can start the translation
process. To do so, open the file with an editor of your choice. Take care
when saving. The file need to be saved in UTF-8 without BOM. If you have
finished translation, copy the files to your Joomla as described above.

If you need any help to do translation package, take the contact for
Alphaplug.com administrator/developer

Please share your work like we do with Alphaplug.com itself and make your
translation available for others who speaking your language. Just post in the
forum and attach your work. Thanks.