Do I understand this ( https://solidproject.org/developers/tutorials/getting-starte... ) correctly: you can only write Solid applications with frontend Javascript?
No, you're looking at a Getting Started Page targeted at web developers, hence they use the tools they are familiar with. These tools take care of auth/data format and other glue code so you can get started easily... But because Solid is based on open standards, you could write a desktop app that makes HTTP requests using your language's clients without issues, but you'll have to write some boilerplate code yourself…
Surely this incarnation of the RDF-based Semantic Web nirvana will succeed where all other attempts have failed! :)