I was a Manjaro user and I could just search and install the languages from AUR, but how can I do it in Bazzite?
For Flatpak, you can configure the localization (spell check and user interface) by using the following command, for example:
flatpak config --system --set languages "en;es;de"
You can then install the languages with the following command:
flatpak update
For applications on the host system, I assume that the only option would be an overlay.
rpm-ostree install hunspell-<languagecode>
(Note: Since most applications are typically installed via Flatpak on Bazzite, this step should not be necessary for the majority of users.)
An alternative could be the plugins from languagetool.org. In addition to spelling, they also check grammar. Plugins are available for Firefox, Chrome, LibreOffice etc.
By default, these send the texts to a server on the internet, which might be a concern for some users. There is also the option to run your own LanguageTool server locally.
The server is available in the following Arch package, for example:
https://archlinux.org/packages/extra/any/languagetool/
It can be run in an Arch container. See:
In the Arch container, the server can be installed and started with the following commands:
sudo pacman -S languagetool
sudo systemctl enable languagetool
sudo systemctl start languagetool
The developer’s site mentions that a homebrew-based installation is also an option. However, with this option I found that the server could not be started after installation.