Switch repo to English, add i18n with EN/DE language files and first-start language selection

- in-game texts moved to lang/en.lang and lang/de.lang (key = text, placeholders)
- game logic emits message keys + args, translated at display time
- localStorage access via own miniquad JS plugin (www/dc_plugin.js)
- code comments, DESIGN.md, CREDITS.md translated; README.md added

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/i18n.rs

@@ -0,0 +1,87 @@
+// Internationalisation: in-game texts live in lang/*.lang files (key = text).
+// English is the reference language and the fallback for missing keys.
+// To add a language, create lang/<code>.lang and add it to SOURCES.
+
+use std::cell::RefCell;
+use std::collections::HashMap;
+
+const SOURCES: &[(&str, &str)] = &[("en", include_str!("../lang/en.lang")), ("de", include_str!("../lang/de.lang"))];
+
+struct I18n {
+    langs: Vec<(String, HashMap<String, String>)>,
+    current: usize,
+}
+
+fn parse(src: &str) -> HashMap<String, String> {
+    let mut m = HashMap::new();
+    for line in src.lines() {
+        let line = line.trim();
+        if line.is_empty() || line.starts_with('#') {
+            continue;
+        }
+        if let Some((k, v)) = line.split_once('=') {
+            m.insert(k.trim().to_string(), v.trim().to_string());
+        }
+    }
+    m
+}
+
+thread_local! {
+    static I18N: RefCell<I18n> = RefCell::new(I18n {
+        langs: SOURCES.iter().map(|(code, src)| (code.to_string(), parse(src))).collect(),
+        current: 0,
+    });
+}
+
+// Available languages: (code, display name in that language)
+pub fn languages() -> Vec<(String, String)> {
+    I18N.with(|i| {
+        i.borrow().langs.iter().map(|(c, m)| (c.clone(), m.get("lang.name").cloned().unwrap_or_else(|| c.clone()))).collect()
+    })
+}
+
+pub fn set_language(code: &str) -> bool {
+    I18N.with(|i| {
+        let mut i = i.borrow_mut();
+        if let Some(k) = i.langs.iter().position(|(c, _)| c == code) {
+            i.current = k;
+            true
+        } else {
+            false
+        }
+    })
+}
+
+#[allow(dead_code)] // used by the settings menu
+pub fn language() -> String {
+    I18N.with(|i| {
+        let i = i.borrow();
+        i.langs[i.current].0.clone()
+    })
+}
+
+// Translate a key (fallback: English, then the key itself)
+pub fn tr(key: &str) -> String {
+    I18N.with(|i| {
+        let i = i.borrow();
+        i.langs[i.current].1.get(key).or_else(|| i.langs[0].1.get(key)).cloned().unwrap_or_else(|| key.to_string())
+    })
+}
+
+// Translate with arguments. Arguments starting with '@' are keys and get translated too.
+pub fn trf(key: &str, args: &[String]) -> String {
+    let mut s = tr(key);
+    for (n, a) in args.iter().enumerate() {
+        let v = if let Some(k) = a.strip_prefix('@') { tr(k) } else { a.clone() };
+        s = s.replace(&format!("{{{}}}", n), &v);
+    }
+    s
+}
+
+// Translate a key in a specific language (used by the language selection dialog)
+pub fn tr_in(code: &str, key: &str) -> String {
+    I18N.with(|i| {
+        let i = i.borrow();
+        i.langs.iter().find(|(c, _)| c == code).and_then(|(_, m)| m.get(key)).cloned().unwrap_or_else(|| key.to_string())
+    })
+}