extconf.rb:36:in `system': Command failed with exit 1: protoc (RuntimeError)
あれれ？
cld3はレポジトリをcloneしてくるだけじゃだめだった気がしてきたね
gem 'cld3', '~> 3.4.2', git: 'https://github.com/zunda/cld3-ruby.git', branch: 'use-ruby-3.1.0', glob: 'intermediate/*.gemspec'
こうかな？
GitHubGitHub - zunda/cld3-ruby: cld3-ruby is an interface of Compact Language Detector v3 (CLD3) for Ruby.cld3-ruby is an interface of Compact Language Detector v3 (CLD3) for Ruby. - GitHub - zunda/cld3-ruby: cld3-ruby is an interface of Compact Language Detector v3 (CLD3) for Ruby.