The plugin:gettext rake task isn't loaded with a full bundler installation, so plugins can't be extracted. Removing the rescue in lib/tasks/gettext.rake shows:

$ rake plugin:gettext[foreman_example]
rake aborted!
LoadError: cannot load such file -- gettext_i18n_rails_js/tasks
/home/dcleal/.rvm/gems/ruby-2.0.0-p353@foreman/gems/activesupport-4.1.5/lib/active_support/dependencies.rb:247:in `require'
/home/dcleal/.rvm/gems/ruby-2.0.0-p353@foreman/gems/activesupport-4.1.5/lib/active_support/dependencies.rb:247:in `block in require'
/home/dcleal/.rvm/gems/ruby-2.0.0-p353@foreman/gems/activesupport-4.1.5/lib/active_support/dependencies.rb:232:in `load_dependency'
/home/dcleal/.rvm/gems/ruby-2.0.0-p353@foreman/gems/activesupport-4.1.5/lib/active_support/dependencies.rb:247:in `require'
/home/dcleal/code/foreman/foreman/lib/tasks/gettext.rake:5:in `<top (required)>'

The layout of gettext_i18n_rails_js changed in 1.x, updated in #7230.