Jinja is one of the most widely used template engines in the Python ecosystem, enabling applications to generate documents by combining dynamic data with templates. Jinja application development is challenging because templates are polyglot artifacts and interact with backend code through implicit cross-language data contracts. However, existing tools provides limited assistance for maintaining consistency the between backend and templates. In this paper, we present OmniJinja, an event-driven VS Code extension for Jinja application development. OmniJinja performs cross-language analysis of backend Python code and Jinja templates to provide backend-aware completion, bidirectional consistency checking, and syntax diagnostics with quick fixes. A lightweight evaluation on three real-world Jinja projects demonstrates that OmniJinja effectively provides backend-aware completion and detects cross-language inconsistencies during Jinja application development.