What is writing a language using letters from another called?

There isn't one specific term for writing a language using letters from another language. It depends on the context and what you're trying to describe. Here are some possibilities:

Borrowing: This is a broad term used to describe when a language takes words, sounds, or writing systems from another language. This could involve using letters from another language to represent sounds that don't exist in the original language.

Adaptation: This refers to modifying an existing writing system to fit the sounds of a new language. This could involve adding new letters or modifying existing ones to represent new sounds.

Transcription: This refers to writing down the sounds of a language using the letters of another language. This is often used for recording spoken language in languages that lack a written system.

Romanization: This is a specific type of transcription where the letters of the Latin alphabet (used in English) are used to represent the sounds of a language.

Transliteration: This is similar to transcription, but it focuses on translating the written form of a language into another writing system, rather than just the sounds.

Orthography: This is a broader term that refers to the conventions of writing a language, including spelling, punctuation, and capitalization. Using letters from another language would be considered a change in orthography.

Ultimately, the most accurate term to use will depend on the specific situation and what you are trying to describe.