polymer 3 how to specify json content-type utf-8

Guys, tell me how to specify the output of json in utf-8. Here is the code

export const fetchCategoryItemsIfNeeded = (category) => (dispatch, getState) => { if (category && category.name && !category.items && !category.isFetching) { dispatch(requestCategoryItems(category.name)); return fetch(`data/${category.name}.json`) .then(res => res.json()) .then(items => dispatch(receiveCategoryItems(category.name, items))) .catch(() => dispatch(failCategoryItems(category.name))); } else { return Promise.resolve(); } }; 

Russian language displays krakozyabami (Thanks in advance!