It has, very significantly. There is still a 日本語-only mailing list (ruby-dev), but the English one has significantly more traffic (ruby-core). No decisions are made in ruby-dev that are not also discussed in ruby-core.

In addition, it's only that mailing list that's split; the bugtracker is in English, the help is all in English (with one or two 日本語 translations).

It's actually never been easier to contribute to Ruby.