I tried to localize the parser strings as well... but that failed the tests because the unit tests don't have a vscode implementation. Any idea on how I could mock that?

I would keep the parser free of vscode-dependencies. It's a bit of duplication but the LanguageValidation-strategy could have a mapping from ValidationError#code to human-readable messages which are then externalized/translated

Originally posted by @jrieken in #132 (comment)