Thank you for submitting your questions.
Please find my comments/answers below:
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Why “1” and “5”? What do these numbers mean?
Answer:
1 refers to the form “one” and “5” refers to the form “other” -
Can I rely on my English-plurals to always be stored under “5”?
Answer:
I am not sure I have understood your question. Could you elaborate a little bit on this? -
I suspect that if my source language had been Russian I would have had other numbers there. Is that correct?
Answer:
Correct! Each language has its own plural rules specified here -
Where can I find some documentation about this?
Answer:
Except for the link provided above, you can also check our documentation guide here and here
I hope this helps.
Please let me know if you need any additional clarification.
Regards,
Nina