The writer is out of luck. A native English speaker will stumble. The letter combination “radchaai” is not merely a non-English word. It is actually an illegal combination of letters in English.
There is no sound corresponding to “aa” in English. There are a small number of words spelled with “aa”. Unfortunately they have varying pronunciations. They are either foreign imports or words with a syllable break between the two letters. The reader confronted with “aa” will pronounce it by analogy, the results depending on what known word with “aa” in it comes to mind. Some possibilities:
- /ɑː/ as in aardvark, bazaar
- /eɪ/ as in Baal (English pronunciation)
- /əʔæ/ as in tetraacetate
- /aʕa/ as in Baal (Hebrew pronunciation)
The combination “dch” is rare in English, but there the writer has a chance. English words containing “dch” are compounds (bedchamber, grandchild, godchild, woodchuck, headcheese, windchill). The reader will tend to see a compound and place a syllable break in the unfamiliar word after the “d”.
If the analogous word that comes to the reader’s mind is foreign sounding (such as Baal), another effect, which we might call “looks foreign”, will cause other parts of the word to pronounced differently also. Some people will read “rad” as /rɑd/ (instaed of /ræd/), and some people will divide “ai” into two syllables (as would be correct in many non-English languages).
There will also be disagreement over where to put stress, the results again depending on the analogy that comes to the mind of each reader. For example:
- 'rad-chaai
- rad-'chaai
- 'rad-cha-ai
- rad-cha-'a-i
If the author cares how this word is to be read by English speakers, it would have been helpful to either provide a pronunciation guide or use a conventional English spelling.
2020 Postscript
The question asked is how a native English speaker would pronounce the name. But a closely related question is how the inventor of the name, Ann Leckie, might have heard the name in her own mind.
The book itself offered no guidance, but after publication Ann posted a statement on the subject. The statement apologized for the difficulties that readers had with pronunciation, and offered guidance.
Her “r” is “like the sound of the letter R”. There is no one way to pronounce the letter “r”. But elsewhere she comments that “however you’re pronouncing things is probably right somewhere”, so she seems to mean the reader to pronounce “r” in whatever way is natural.
Her “a” and “i” are like those of Romance languages – which is to say, like the “a” in “palm” or “bra”, and the “i” in “pizza” or “casino”. Her “ai” is two separate sounds: “a” followed by “i”, again as often seen in in Romance languages. Her “aa” is like “a” but longer.
And she hears this particular name with a stress on the first syllable. Taken all together, I guess the author might be hearing something like:
- /'rad.tʃaːi/ (RAHD-chaah-ee)