Konverter: a multi-tenant SaaS for converting, translating and exporting business spreadsheets

Our client, [Eksterna DOO](https://eksterna.rs) from Zaječar, is an accounting, tax advisory and legal services firm, and the platform's owner and first operator. Their teams prepared product and document spreadsheets by hand: converting Cyrillic to Latin, translating content and reshaping columns for import into a target system. The work was repetitive and error-prone, and it had to be offered to other companies as a paid service, with proper accounts, invoicing and Serbian payment standards.

We designed and built Konverter, a multi-tenant SaaS on Cloudflare's edge. Users upload CSV/XLSX files, preview and edit cells in the browser, run Cyrillic→Latin conversion and AI-assisted translation, then export in the required import format. Companies register with their PIB/MB, invite team members and subscribe through a pro-forma invoice → bank transfer → approval workflow, including NBS IPS QR payment codes. Public pricing: Basic 34,899 RSD/year and Pro 69,999 RSD/year (excl. VAT), plus pay-as-you-go on request. **Engineering quality:** JWT in HTTP-only cookies, CSRF tokens, RBAC, rate limiting, input sanitization, parameterized SQL, request size limits and file content encrypted at rest, with versioned D1 migrations, separate staging and production environments and Playwright test suites for security, billing, i18n, accessibility and performance.

1. Upload CSV/XLSX, preview multiple sheets, save drafts and edit cells in the browser. 2. Convert Serbian Cyrillic to Latin and translate content with an LLM. 3. Export to the target import format (Taurus) per company. 4. Multi-tenant organizations (PIB/MB) with roles, invitations and audit logs. 5. Trial, annual plans and pay-as-you-go usage tracking. 6. Pro-forma and invoice PDFs, NBS IPS QR codes, NBS exchange rates, payment proof upload and admin approval. 7. Admin panel for users, companies, files, invoices, analytics and support tickets, with Telegram notifications for operators. 8. English, Serbian and Chinese interface, dark mode and an installable PWA.

- Cloudflare Workers, D1, R2, Pages, Turnstile - React 19, React Router, Tailwind CSS 4, i18next, Workbox (PWA) - SheetJS (xlsx), qrcode - OpenRouter (LLM), Resend (email), Google Sheets API (invoice numbering), NBS APIs - Playwright end-to-end tests

1. In production on Cloudflare and used by Eksterna DOO as platform owner and first operator. 2. A manual, error-prone spreadsheet routine became a repeatable upload → convert → translate → export flow. 3. Ready to be sold to other companies as a paid service, with accounts, invoicing and Serbian payment standards built in.