"Whose water is it?"
Translation:Чья это вода?
Russian rarely uses questions like "Whose is this water?" Normally it is similar to "Whose water is this?", with unchanging «это» between the question word and the object.
So it's always "это", whatever the gender of the noun it relates to? Is this a general rule or just a rule for this specific question?
My first language is also a Slavic one and this just sounds so... wrong to me. Why would you not change the ending of "это", especially in the written form of communication?
Because it's an absolute rule. All languages have this kind of grammar features.
Why does the word "чья" have a soft sign if the vowel я already makes ч soft? Why is it not spelled "чя"? or do I misunderstand how the soft sign works
The spelling follows the pattern used i Russian when the consonant is soft but the vowel letter retains its Й sound.
We never write чя, but is would be pronounced same as /ча/ because Ч is always palatalized.
чья is pronounced /чйа/.
It isn't. At least, normally it is an appropriate form of чей + "это" + the noun.
It should be «эта». «это» is not natural in this context.