You can use the online Rosetta Launchpad tool to help translate Avogadro. These translations are periodically collected and merged into the development repository for new releases.
There are two translation template. One (the "libavogadro" template) is for the core Avogadro library, main extensions, tools, and other plugins. The other is for the Avogadro application.
There is a mailing list, avogadro-translate to discuss translation and localization of Avogadro. This is an excellent place to ask questions about the meaning of translation strings, suggest word changes, etc. We are very open to improving Avogadro and making it easier for translation and localization worldwide.