First of all, I assume that with “largest dictionary” you mean the dictionary with the largest number of entries (lemmas), not the one that fills the largest number of pages. In this form, the question probably cannot be answered, given the fact that different languages have different systems of lemmatisation.
To begin with, Wiktionary includes proper nouns, while traditional dictionaries exclude proper nouns. The Dutch WNT covers texts down to 1921 and has 430000 entries. The second edition of the OED (1989) has 291000 entries, but the current on-line version claims to have 600000 words.
Just to problematize the question I would like to mention Arabic, a language considered to have a very large vocabulary. Arabic dictionaries are arranged according to roots. The numerous verbal derivatives of any given root are all counted as a single lemma, while the singular nouns derived from a given root are treated as separate lemmas. The largest dictionary of Classical Arabic, the Tāj al-ʻarūs, is said to contain 120000 “words”. This looks a lot less than the WNT and the OED. However, the Tāj was compiled in the 18th century, the other two in the 20th; the vocabularies of Dutch and English have obviously increased enormously in the last two centuries. Moreover, the Tāj contains in principle only the words found in the Qur’an (not many) and in the early Arabic poets, so let’s say words attested in texts from before about 700 AD. I doubt whether there are many other languages that clock up such a big number by such an early date.
