Why "la salle de bains" and not "la salle de bain" ?

Question

Your translation corrects "la salle de bain" to "bains" yet other translator translations seem to use either ???????

Maarten Kamp · Accepted Answer

Stephen, 
it looks like you have repeated your question from 2 weeks ago ?
Link to Cécile’s response : 
https://progress.lawlessfrench.com/questions/view/why-la-salle-de-bains-and-not-la-salle-de-bain
 ‘ Bain ‘ is not “ bathtub “, that is ‘ la/une baignoire ‘. 
“salle de bains“ does not translate as ‘ room for/of bath(tub or tubs) ‘ but is the word for the room in which baths (as in washes) are taken.
For standard French metropolitan usage of individual words, and often also for expressions, it is better to use traditional reputable ( metropolitan) French dictionary sites. 
Of course, ‘ standard ‘ usage is not necessarily exclusive - many of the sites I found that used ‘ salle de bain ‘ seem to be Canadian so there may be a regional difference. Metrolopolitan French is routinely used on this site.
https://www.larousse.fr/dictionnaires/francais/salle/70707#locution
 https://dictionnaire.lerobert.com/definition/salle 
https://www.dictionnaire-academie.fr/article/A9S0264

Céline Pickard · Answer

Bonjour Stephen,
Maarten's explanation is excellent!
In French, you will find many expressions with "bain/bains":
- bain is in the singular form with the following ones:

un maillot de bain = a swimsuit
un peignoir de bain = a bathrobe
une serviette de bain = a bath towel
des sels de bain = bath salts

-> they are used for/after bathing
 
- bains is in the plural form with the following ones:

une salle de bains = a bathroom
un établissement de bains = a bathhouse

-> they are places where baths (as in washes) are taken
 
I hope this is helpful.
Bonne journée !