I'll mention a few languages: I speak Greek (funny alphabet, articles come before everything, a few grammatical cases, a quadrazillion different combinations for verbs), Finnish (declense absolutely everything, fif-friggan'-teen grammatical cases, vowel harmony, etc.), and Gaelic (letters are pronounced following an obscure, sometimes arbitrary pattern and some can be skipped if you remember which, when and why), so the Slavs don't frighten me. But I wasn't thinking of Bulgarian yet. Perhaps SerboCroatoslavian, or Russian.