Jyst (in many places jest) is completely assimilated into native Welsh, and is one of those words that will definitely make you sound more like a native if you use it! For example,
o'n i jest isio gwybod...
I just wanted to know...
sounds thoroughly authentic, I would say.
When just means only, dim ond is fine however - though often shortened to 'mond.
'mond punt sy gen i
I've just got a pound