The Italian words that we use in music only exist because of the classical canon and the dawn of standardized Western staff notation. The Italian counterparts of these straightforward words are only relevant in a classical or jazz setting where sheet music is the staple. Outside of that, say in a popular setting, there is no need for them. Speed up, and slow down are pretty clear and need no explanation. In fact, in popular musics, these Italian terms are just simply not used. Many times there are other words used to replace these Italian ones, but there are also terms that are exclusive to popular styles.