1. The TARDIS's translation capabilities are limited:
While the TARDIS is capable of translating languages, it's not a perfect system. It primarily relies on the Doctor's input and understanding of the language. There's a clear distinction between a language the TARDIS is familiar with and one it hasn't encountered before. The Judoon's language, though somewhat similar to English, might be sufficiently different to pose a challenge for the TARDIS's translation mechanisms.
2. The Judoon's language is intentionally obscured:
The Judoon are portrayed as a proud and somewhat militaristic species. They prefer to maintain their unique language as a symbol of their identity and power. Their language likely serves to maintain their sense of solidarity and distinguish them from other species.
3. It adds to the mystery and tension:
The fact that the TARDIS cannot translate the Judoon's language creates a sense of intrigue and suspense. It forces the audience to rely on context clues and the Doctor's limited understanding to decipher their intentions. This adds a layer of complexity to the story and makes the Judoon appear more formidable and alien.
4. It's a deliberate choice by the writers:
The writers may choose to keep the Judoon's language untranslated for dramatic effect. It emphasizes their alien nature and makes them more memorable. This decision also allows for some humour, as the Doctor and companions struggle to understand their pronouncements.
Ultimately, the TARDIS's inability to translate the Judoon's language is a creative decision that serves both narrative and thematic purposes. It adds to the mystery, complexity, and overall enjoyment of the show.