The English copycat term 'verjuice' always sounds a little icky and bodily.

I had to look the Spanish up - it's 'agraz' (as in agrio/aigre)

'Mosto' is the standard name for 'grape juice for winemaking', which is another problem one, as who in Britain knows what 'must' is? Spanish winemakers and restaurateurs are also keen on 'caldo' ('broth') as a metaphor for 'wine', to avoid repeating the same word, which is another stumbling block. Because we can use 'brew' to refer to tea or beer (proper British drinks!), but don't have a readymade equivalent for wine (foreign muck!)