"Where is the bathroom?"
No. In ex-USSR, toilet room and bathroom are usually different rooms, and former is called 'туале́т', the latter is 'ва́нная'.
Russian nouns have several case forms. «Туале́т» is the nominative case form, it's used for grammatical subjects (like in this sentence). «Туале́те» is the prepositional case form, it's used after certain prepositions.
Here you are.
Nominative case (имени́тельный паде́ж) is used for the grammatical subject of the sentence, someone ‘doing the action’. It often corresponds to the first noun in the English sentences.
- Я чита́ю. ‘I’m reading’ (я is nominative, I’m doing the action of reading)
- Ты идёшь. ‘You’re going’ (ты is nominative, you’re doing the action of going)
- Де́вочка игра́ет. ‘The girl is playing.’ (девочка is nominative, it does an action of playing)
- Сто́л стои́т вон там. ‘The table is standing over there.’ (стол is nominative, it does an action of standing, i.e. being located in its default position)
- Высокотехнологи́чный япо́нский туале́т обеспе́чивает высоча́йший у́ровень комфо́рта. ‘A high-technology Japanese toilet provides the highest level of comfort.’ (туалет is nominative, it does an action of providing)
Another use of nominative is ‘X is Y’ sentences, where both X and Y are in nominative case:
- Я программи́ст. ‘I’m a programmer’ (both я and программист are nominative case)
- Сто́л чи́стый. ‘The table is clean.’
- Туале́т но́вый. ‘The toilet room is new.’ (both туале́т and но́вый are nominatives)
Genitive case (роди́тельный паде́ж) is used to show possessor, and in such meaning it rougly corresponds to English ’s or preposition ‘of’:
- кни́га де́вочки ‘girl’s book’,
- кра́й стола́ ‘edge of the table’,
- цена́ туале́та ‘price of the toilet’
It is also used in conjunction with «нет» to express absence of something:
- нет кни́ги ‘[there] no book’,
- нет туале́те ‘[there] no toilet’.
It can be used instead of the accusative case for a direct object. Direct object is something that is affected the most by the action (e.g. in ‘I see a dog’, a dog is a direct object). Genitive can be used for direct objects in 2 cases:
- in negative sentences (most commonly with abstract things; never with living beings; the full rules for choosing between accusative and genitive are quite Byzantine and can be ignored in most cases ^^"),
- to mean ‘some’: я вы́пила со́ка ‘I drank some juice’ (as opposed to я вы́пила сок ‘I drank the juice’).
Some prepositions require genitive case:
- из туале́та ‘from the toilet’.
Dative case is used for a person or thing benefitting from the action, and it’s often translated with the English ‘to’:
- де́вочка дала́ мне кни́гу ‘the girl gave me a book’ (мне is dative of я),
- ма́ма рассказа́ла де́вочке ска́зку ‘mum told the girl a fairy tale’ (де́вочке is dative of де́вочка).
Some prepositions require dative case:
- к туале́ту ‘to the toilet’.
Accusative case is used for the direct object of the sentence. Direct object is someone or something directly affected by the action. For example, in ‘I see an apple’, ‘apple’ is the direct object.
- Я чита́ю кни́гу. ‘I’m reading a book.’ (кни́гу is accusative case)
- Я купи́л себе́ высокотехнологи́чный япо́нский туале́т. ‘I’ve bought a hi-tech Japanese toilet for myself.’ (туале́т is accusative case)
Some prepositions require accusative case. Many prepositions have double usage: when used with accusative, they indicate the direction, final point of destination. When used with another case (usually prepositional), they indicate the location:
- в туале́т ‘into the toilet [room]’ (туале́т is accusative; compare в туале́те ‘in the toilet room’).
Instrumental case is used to indicate the instrument, and is translated ‘by the means of’, ‘by’ or ‘with’. For example:
- Я пишу́ письмо́ ру́чкой ‘I’m writing the letter with a pen’ (ру́чкой is instrumental, it’s a means to write a letter)
- Рестора́н «Рэсуто Удзё» удивля́ет госте́й свои́м туале́том, офо́рмленным в ви́де япо́нского са́да. ‘The restaurant Resuto Ujō surprises its visitors with its toilet room, decorated as a Japanese garden.’ (туале́том is instrumental case, it’s a means to surprise the visitors)
Some prepositions require instrumental case:
- ма́ма с до́чкой ‘mother with a daughter’ (note that this is a different ‘with’: «мама с дочкой» doesn’t mean that mother is using a daugher as an instrument, it means ‘mum together with the daughter’; those meanings are separate in Russian).
Prepositional case is only used with some prepositions:
- в туале́те ‘in the toilet room’,
- о туале́те ‘about the toilet room’.
It is never used without a preposition.
The Russian presumably comes from the French. The pronunciation of "туалет" involves the vowels "ua" or "oo-a" as in the French "toilette". The English "toilet" also comes from the French but from an earlier period when the "oi" vowel in French was still pronounced as "oi" rather than "wa".
American English is a bit strange in this fact, as the room can contain no bath or the facility for you to bathe but they still call it a 'bathroom'. As Duolingo is trying to teach people from different cultures and countries it would be better if they did not use bathroom but accepted it as an answer. If they want to ask the question in English and need to keep the American users of Duolingo happy, they could use 'restroom', which would be understood by those who speak American English but would not confuse those of us that speak the Queen's English.
- Ва́нная is used for rooms where a bathtub (ва́нна) is situated. A room with a shower might work, too.
- Туале́т is used for rooms where a toilet bowl (унита́з) is situated.
If you have both a toilet and a bathroom/shower in the same room, then you can use them interchangeably.
'Bathroom' does not mean 'toilet' in British English. I had to put my mouse over the word because I thought it must refer to a Russian word I hadn't learnt yet. Perhaps if you put "bathroom/toilet" rather than just "bathroom" here? After all it seems (from someone else's post) that there are two words in Russian just as in British English.
I don't know what you mean by "Both are accepted". It's not a question of what English words are accepted from a learner as the English for 'туалет'. It's a question of what English word or words are used by Duolingo to ask to be translated as 'туалет'. If a British person has learnt 'туалет' they will think of it as 'toilet', especially if Duolingo used that English translation (which it did). If Duolingo then asks to translate 'bathroom' the British person will not think of 'туалет'.