Can you name some other languages that use mutations like the Welsh language does?

All of the Celtic languages use "mutations" - though the way they are used and the type of mutation is different in each language; for example, Welsh has three types of mutation: aspirate, nasal, and soft; Breton has four types of mutation: soft. hard, aspirant and "mixed"; Irish and Manx have two - "aspirate" and "nasal" - although in the case of Irish the terms "lenition" and "eclipsis" are genrally used. Scottish Gaelic has only one type of mutation: "lenition" (some older people - e.g. my grandmother, still used the term "aspiration").