Windows 10 Multi-Session images only come with English pre-installed. To support other languages for your AVD users, you will need to install the language packs directly on the Image VM that your host pools will be based on.
Warning: Installing language packs incorrectly via other means, such as pre-existing GPOs can cause Sysprep to fail. Language packs must be installed in a manner that Windows 10 multi-session supports.
Nerdio recommends following a similar procedure to Microsoft's process as defined here: https://docs.microsoft.com/en-us/azure/virtual-desktop/language-packs.
with the exception that instead of initiating Sysprep on the image VM itself, to initiate it from the Nerdio Manager Images page.
Tip: For the fileshare requirement (https://docs.microsoft.com/en-us/azure/virtual-desktop/language-packs#create-a-content-repository-for-language-packages-and-features-on-demand) you can add a file share to the existing storage account you may be using already for FSLogix.
This can be useful to keep resources consolidated, and allows you to simply use permissions you already have granted for FSLogix on the storage account.