[ehcrea]

Requested by Eren · project thread

What changes

• A language menu in the header (English, Deutsch, Nederlands, Türkçe; short codes on phones). The choice is saved in the browser, and English stays the default.
• Everything is translated: interface, calculator names, input labels and options, result rows, warnings, formulas that contain words, sketch labels and animation captions, the theory "?" notes, the material table, the Saved dialog and the PDF report. Dates follow the language.
• js/i18n.js adds PEC.t(englishText, vars). js/i18n/de.js, nl.js and tr.js map each English text to its translation (686 texts each). Untranslated text falls back to English.
• A few sketch notes and captions were built by joining pieces of text. They are now whole sentences or templates, so they can be translated.

Tests

• npm test now also checks that every text the app can show exists in each language and keeps its {placeholders}. It also renders every calculator, option and sketch phase in each language and fails if any text shows up untranslated. 726 tests pass.
• I checked it in headless Chromium: switched all 17 pages in each language with no console errors, exported a PDF in each language (umlauts and Turkish letters render with the bundled DejaVu font), confirmed the choice survives a reload, and checked the phone layout.

Notes: numbers keep a decimal point in every language, to match the result values. The source-article link titles stay in English because they are the real JPE article names.

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