feat(i18n): add internationalization support (English + German)

[Marcel-Holzhauer]

Summary

This PR adds internationalization (i18n) support to swiML. The rendering language is now determined by an optional xml:lang attribute on the <program> element. English and German are supported as initial languages.

Changes

New files

• i18n/en.xml — English translation file (terms, labels, date format)
• i18n/de.xml — German translation file

Modified files

• swiML.xsd — Added optional xml:lang attribute on <program>
• swiML.xsl — Externalized translations, dynamic alignment calculations, lang attribute on HTML output
• swiMLPT.xsl — Externalized translations, fixed "Snorkle" typo
• swimTraining.xsl — Externalized translations
• README.md — Added xml:lang to usage example, added "Language Support" section

Usage

<!-- German output -->
<program xmlns="https://github.com/bartneck/swiML" xml:lang="de">
    ...
</program>

<!-- English output (default, also when xml:lang is omitted) -->
<program xmlns="https://github.com/bartneck/swiML" xml:lang="en">
    ...
</program>
Design decisions
- Language source: Determined by xml:lang on the source document, not by an XSLT parameter. The author decides the language when writing the program.
- Backward compatible: Existing documents without xml:lang continue to render in English.
- External translation files: Translations are stored in separate XML files (i18n/{lang}.xml) loaded via document(). This keeps the XSLT clean and makes adding new languages trivial.
- Dynamic alignment: The character-width alignment calculations now use string-length() on translated labels instead of hardcoded constants, so alignment works correctly regardless of language.
- Fallback: If a translation key is missing, the raw key name is displayed as a debugging aid.
Adding a new language
Create i18n/{code}.xml following the structure of en.xml or de.xml. No other changes required.
Backward compatibility
- Existing swiML documents are not affected — they render identically to before.
- The XML schema change is purely additive (optional attribute).
- No changes to the Python library.