Based on the capabilities and flaws of Transifex the following approach seems best suited:

Setup:

• Create a project getsimple
  • license = other open source
  • source language = en_US
  • maintainers = GetSimple developers/moderators and main plugin developers
  • tags: opensource, getsimple
• Goto Manage/Access Control and set
  • project type = project hub
  • access control = limited access (teams)
• Add a resource lang and upload the english language file
• Create additional languages for the available languages and upload the files
• Always create the no-country language, too, e.g. en

Management:

• translators are added on request for their language (and the respective language without country code)
• developers download files for releases

Setup:

• (the plugin developer registers himself with Transifex)
• create a project with name getsimple_plugin_name, e.g. getsimple_i18n_gallery for the I18N Gallery plugin, which uses a plugin file named i18n_gallery
  • license = (either license)
  • source language = (any english variant)
  • maintainer = (plugin developer)
  • tags: opensource, getsimple, …
• Goto Manage/Access Control and set
  • project type = outsourced project
  • outsource access to = getsimple
• Add a resource lang and upload the english language file
• Add new translations by uploading existing language files

Management:

• no user management needed - any translator registered with the GetSimple project can translate any plugin (to the language(s) he is registered for)
• developer downloads files for plugin releases or
• uses a enhanced Translate plugin: download language files, compare them with current state, save them, test them, update them and upload them to Transifex

Advantages

• Management of translation team centralized - a german translator can translate any plugin
• consistent tagging - translator can search for tag getsimple and easily translate all available plugins - this could greatly improve language availability
• consistent naming of projects allows automated access
• because of this an enhanced Translate plugin can simplify Testing of translations
• for users registered with Transifex: it's easy to find all available translations of a plugin and download one

Disadvantages

• if language files are only stored on Transifex (until they are included in the plugin), a user needs to register with Transifex to download them

Problems:

• Transifex does not show the technical key for the translation, although this can convey important information: e.g. if you have 2 items TAB_XXX and TITLE_XXX which are the same in english, but should be different in another language because the translation is rather long and thus should be shortened for the tab.
• When downloading, Transifex replaces non-translated items with the source language text - the Translate plugin uses a hack to recognize this