Translate remaining German content to English

Finishes the project-wide English convention:
- CLAUDE.md (the user opted in to translate it too).
- examples/hello.ail.json: "Hallo, AILang." -> "Hello, AILang.".
- E2E test assertion and IR snapshot for hello regenerated to match.
- DESIGN.md "Project language: English" section: the previous
  CLAUDE.md exception is dropped.

After this commit, no German remains in any committed file
(grep -P '[äöüÄÖÜß]' is empty across .rs / .md / .json / .ll).

Verified: cargo test --workspace passes (44/44).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/DESIGN.md

@@ -47,12 +47,11 @@ here as soon as they are established.
 
 All in-tree content is written in English: source code (identifiers,
 comments, string literals, CLI help), design documents, the journal, agent
-prompts, READMEs, commit messages, and new examples. The only exception is
-`CLAUDE.md`, which is the user's own instruction file. The live conversation
-between user and me stays German for ergonomic reasons; everything that
-lands in git is English. This keeps diffs and tooling output uniform and
-matches the audience for AILang (LLM authors), for whom English is the
-default.
+prompts, READMEs, commit messages, examples, and `CLAUDE.md`. The live
+conversation between user and me stays German for ergonomic reasons;
+everything that lands in git is English. This keeps diffs and tooling output
+uniform and matches the audience for AILang (LLM authors), for whom English
+is the default.
 
 ## Decision 1: source = data, not text