It's neither.
Sapkowski invented his own elven language which isn't related to sindarin or quenya, which were invented by Tolkien.
Sometimes it seems to borrow words from sindarin. (Like Lembas
)
I think Sapkowski used a lot of old middle-english and saxon words, combined with some older goidelic (Celtic) grammar rules.
Next to English I also speak Dutch, and a lot of the elvish words are very similar in pronunciation and meaning to older Dutch and Middle English equivalents.