I have worked over the requirements shared by you and like share that at present the requested support for language and spell checking is unavailable in Aspose.Slides. I have created an issue with ID SLIDESNET-35709 in our issue tracking system to further investigate the possibility of implementing the requested support on our end. This thread has been linked with the issue so that you may be automatically notified once the support will be available.
I have verified from our issue tracking system and regret to share that at present the requested support is unavailable in Aspose.Slides for .NET and concerned issue is unresolved. We will share the good news with you as soon as the support will be available in Aspose.Slides.
We are sorry for your inconvenience,