Add multi-language support

[Coelopsychis]

• Add initial multi-language support with German and English locale files.
• Introduce centralised translation helper using txt(...) with fallback to the default language.
• Replace hard-coded UI strings across the Streamlit pages with translation keys.
• Add language-aware labels, headings, buttons, status messages, warnings, errors, and charts.
• Add language-aware topology images.
• Internationalise generated HTML report.
• Move tooltips into locale files using the tooltips.* key namespace.
• Keep technical data keys stable while translating only UI strings.
• Add locale mappings for energy system unit parameterisation.
• Preserve existing German behavior as the default language.
• Add caching to prevent the app from being slowed down by translations.

All hard-coded UI strings have been replaced by keys in the format namespace.optional_subgroup.keyword. Some commonly used phrases have been collected using a common.* namespace to reduce redundancy.
The language can now be selected in helpers.py by setting DEFAULT_LANGUAGE to either de or en, or by setting st.session_state['language'] in the future. Because the localisation data is cached, Streamlit has to be restarted after modifying the locale JSON files.