Generally, everybody tells us their own native language/ the language they teach for their living is the hardest to learn. Why? People think it is an indicator for the worth of the culture or the difficulty of their job. Difficult language = developed culture.
That of course is a delusion. Languages in the course of evolution become easier and easier. Just think about Sanskrit or ancien Greek.
Chinese is difficult because of it´s pictograms, but the grammar is almost nonexistent.
Russian and Polish have a highly complicated grammar and are very hard to pronounce without sounding like a nutcase for western speakers. Believe me, been there, done that, in both countries.
German is pretty difficult, but nothing supernatural.
Spanish is easy. Of course, the spanish tell us that "it has a very difficult grammar". Ridiculous.