Apple has received much criticism for how Siri seems to lag behind more modern competitors like Amazon Alexa and especially the new Google Assistant, but also Microsoft's Cortana are better at some things.
One area where Apple on the other hand in the forefront is the language support. Siri is available in 21 languages in 36 countries. Cortana supports 8 languages in 13 countries, Google Assistant only four languages, and Alexa understands only English and German. Siri is the only assistant who understand Swedish.
In an interview with Reuters describing Apple's head of speech Alex Acero how the company builds up support for new languages.
It starts with letting a group of speakers of the language in question read a series of texts with different dialects and accents. These form the basis for a new language model which can then be improved in stages.
Before it is time for Siri Apple adds support for dictation. Recordings from there then sent anonymous to Apple, from users who have approved it, and helps to improve the language model's ability to recognize the right word. Finally hired voice actors to give voice to Siri in the new language, before it rolls out in an iOS update.
New languages do not support all of the Siri functionality from the beginning, but it builds up little by away.