English is an indo-european language, a germanic/romance hybrid to be precise (old english is the germanic bit, norman french and latin are the romance parts).
It is common for non-native speakers of english to think that the relative paucity of inflected endings means that english grammar is "simple." This is why such people speak english so poorly.
There's a world of difference between being minimally intelligible in english (which is admittedly easy compared to other languages with more inflected word endings), and speaking english grammatically correctly. Deriving the correct meaning from colloquial english speech is harder still than speaking grammatically correctly, and this is what Siri manages to do remarkably well.