Music scores may use many languages. The most familiar musical directions (mezzoforte, crescendo, ritardando, etc) are Italian, and these seem to have stood the test of time. However, to varying extents, composers have used their own native language in scores. An extreme case was Percy Grainger, who invented a new 'English' language of his own, but most people compromise in favour of intelligibility. The key thing is to be consistent and if necessary include a set of explanations at the start of the score. In this manual, UK English has been used where appropriate, but on the whole the current conventions have been observed. Alternative names in different languages have also been given.
Phonetics can be useful, especially in certain kinds of vocal music, since these deal purely with the sounds of words rather than their spelling or meaning. The International Phonetic Alphabet represents a standardised listing of these, and the International Phonetic Association website includes downloadable files of the phonemes in a variety of of languages.