I found out the solution. You have to expect T to be an array, not element of an array.

type TTranslationResult<T> = T extends (number | string)[]
  ? string
  : ReactNode | ReactNode[];

export default function translate<T extends ReactNode[] = string[]>(
  key: TDictionaryKey,
  ...params: T
): TTranslationResult<T> {
{

...

}