This is called “grammatical number”.
In English, there is singular and plural.
In English, the rule is simple: If the number is exactly 1, it is singular (“one” in Transifex). Otherwise, it is plural (“other” in Transifex).
Grammatical number rules may be different for each language. I don't know anything about Russian, but it could be that Russian has a different grammatical number system than English. Which would mean it has more than just singular and plural.
To know what exactly these rules are, you must know Russian a little bit or ask someone who knows the language better than I do.