Translation:She was putting the plates on the table.
Did you report it? http://context.reverso.net/translation/french-english/mettait
Did you say "She was placing the plates on the table?"
What was your complete sentence? "She had the plates on the table." would be wrong. "She had put the plates on the table." would be the pluperfect tense in French and not the imperfect. "Elle eût mis les assiettes sur la table." http://conjugator.reverso.net/conjugation-french-verb-mettait.html
You could try reporting it or "She was setting the plates on the table." might be better suited to imperfect. http://context.reverso.net/translation/french-english/mettait
“Dish” has more than one meaning though. If it is a dish used to serve food to everyone or if it is referring to the course being served, then it is “plat”, but when you are talking about putting the dishes on the table, you could mean dishes for eating or plates, which would be “assiettes.” https://dictionary.reverso.net/french-english/Plat https://dictionary.reverso.net/english-french/Dish
“la vaisselle is a collective word for “the dishes”. We use it in the expression “to do the dishes” = “faire la vaiselle”.
Well, if she was doing it then I would use the article in English. In French they also use the definite article for a generalization where we would not use the definite article, but then you would not use the continuous form of the verb. “One puts plates on the table.” is an example of a generalization, but it is not a translation of this sentence.