So in spanish, it's spelt the same way, but different pronunciation.
Here's how you would pronounce isaac in spanish: eee-sock (I know it sounds silly, but if you say it, that's how you pronounce it in Spanish.) When a name starts with "I", you don't say it like how you say in english, you kinda have to "pretend" it's an "e" .
There will be some english names that are spelt the same way (or spelt differently- it depends) and pronounced differently.
Hope this helps!!