spelllang options are local options and by default sessions only store global options.
You can change this behavior by updating
sessionoptions to also include
The reason why local options are typically not stored in a session file is that they're normally managed by per-filetype plugins, and those plug-ins will be reloaded when the session is restored. Not storing local options gives the plug-ins the opportunity to set options differently, which might be useful or relevant when restoring a session after upgrading plug-ins.