Synonyms of mountainous in English:

hilly, craggy, rocky, alpine, high, steep, precipitous

upland, highland

huge, enormous, gigantic, massive, very big, very large, great, giant, colossal, mammoth, vast, immense, tremendous, mighty, formidable, staggering, monumental, Herculean, epic, prodigious, monstrous, titanic, towering, king-sized, king-size, gargantuan, substantial

informal mega, monster, whopping, whopping great, thumping, thumping great, humongous, jumbo, hulking, astronomical, astronomic

British informal whacking, whacking great, ginormous