Translate project content to English

Make English the project-wide language: all comments, string literals,
CLI help text, design docs, journal, agent prompts, and README. Only
CLAUDE.md (the user's own instruction file) stays German, and the live
conversation between user and Claude continues in German.

Adds a new "Project language: English" section to docs/DESIGN.md as
the durable convention. No logic changes — translation only. Four
internal error strings in the typechecker were retranslated; no
test asserts on their wording.

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

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

crates/ailang-check/tests/workspace.rs

@@ -1,8 +1,8 @@
-//! Integrationstests für `check_workspace` (Iter 5b).
+//! Integration tests for `check_workspace` (Iter 5b).
 //!
-//! Diese Tests fahren über die kanonischen `examples/ws_*.ail.json`-Files;
-//! der Loader und der Checker zusammen sind die Pipeline, die `ail check`
-//! im JSON-Modus auf einen Workspace wirft.
+//! These tests drive the canonical `examples/ws_*.ail.json` files;
+//! the loader and the checker together form the pipeline that `ail check`
+//! runs in JSON mode against a workspace.
 
 use ailang_check::{check_workspace, Severity};
 use ailang_core::load_workspace;
@@ -15,8 +15,8 @@ fn examples_dir() -> std::path::PathBuf {
 
 #[test]
 fn happy_path_resolves_qualified_import() {
-    // ws_main importiert ws_lib und ruft `ws_lib.add` auf — vollständig
-    // typisiert. Erwartet: keine Diagnostics.
+    // ws_main imports ws_lib and calls `ws_lib.add` — fully typed.
+    // Expected: no diagnostics.
     let entry = examples_dir().join("ws_main.ail.json");
     let ws = load_workspace(&entry).expect("load ws_main");
     let diags = check_workspace(&ws);
@@ -29,14 +29,14 @@ fn happy_path_resolves_qualified_import() {
 
 #[test]
 fn unknown_import_is_reported() {
-    // ws_broken referenziert `ws_lib.bogus` — Modul ist da, Def nicht.
+    // ws_broken references `ws_lib.bogus` — module is there, def isn't.
     let entry = examples_dir().join("ws_broken.ail.json");
     let ws = load_workspace(&entry).expect("load ws_broken");
     let diags = check_workspace(&ws);
     assert_eq!(diags.len(), 1, "got: {:?}", diags);
     assert!(matches!(diags[0].severity, Severity::Error));
     assert_eq!(diags[0].code, "unknown-import");
-    // Kontext muss strukturiert auf Modul + Defname zeigen.
+    // Context must point structurally to module + def name.
     assert_eq!(
         diags[0].ctx.get("module").and_then(|v| v.as_str()),
         Some("ws_lib")
@@ -49,7 +49,7 @@ fn unknown_import_is_reported() {
 
 #[test]
 fn unknown_module_prefix_is_reported() {
-    // ws_unknown_module hat keine Imports, referenziert aber `nope.x`.
+    // ws_unknown_module has no imports but references `nope.x`.
     let entry = examples_dir().join("ws_unknown_module.ail.json");
     let ws = load_workspace(&entry).expect("load ws_unknown_module");
     let diags = check_workspace(&ws);
@@ -64,10 +64,10 @@ fn unknown_module_prefix_is_reported() {
 
 #[test]
 fn invalid_def_name_with_dot_is_reported() {
-    // Synthetic: ein Modul mit einer Def, deren Name einen Punkt enthält.
-    // Wir konstruieren das als Module direkt und füttern es in einen
-    // Trivial-Workspace, weil die kanonische Konvention das gar nicht
-    // erst auf Disk durchlassen sollte.
+    // Synthetic: a module with a def whose name contains a dot.
+    // We construct this as a Module directly and feed it into a
+    // trivial workspace, because the canonical convention should not
+    // let this through to disk in the first place.
     use ailang_core::ast::*;
     use std::collections::BTreeMap;