When installing from sources, a simple
make install cd intl make install
should completely install the documentation tool.
When installing from a archive with the binaries, it should be sufficient to copy the binaries to a directory in the PATH.
To have fpdoc available in several languages, the language files should be installed in the following directory on Unix systems:
/usr/local/share/locale/XX/LC_MESSAGES/
or
/usr/share/locale/XX/LC_MESSAGES/
Depending on the setup. Here XX should be replaced by the locale identifier.