You can create inserts in any language you like. The text you provide when creating an insert is treated as a literal string. If you need to provide an insert in multiple languages, do the following:
1. Create an insert per language.
2. Define a custom audience and use the Language filter under the Device tab to ensure the insert is delivered to the appropriate audience.
In the future, Insert will include improved localization capabilities to prevent the need to create multiple inserts.