My open source Android application has internationalization done the Android way, with strings.xml files.
The community has many people from many countries, and they are willing to contribute/improve translations using a collaborative website.
There is Launchpad but it only supports the gettext format so we would have to use scripts, not very convenient.
What website is convenient for translating open source Android applications?
Anybody must be able to start contributing without any sort of prior approval.
One tool was not yet listed so far - Weblate. It has native support for Android string resources and is already used for several apps (for example OsmAnd). Though it requires you to use distributed version control system (preferably Git).