Based on the capabilities and flaws of Transifex the following approach seems best suited:
Translations for GetSimple
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
Translations for Plugins
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/Disadvantages of this Setup
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