Changing the language directly within Storybook is not supported.
To preview your component in different languages, create separate stories for each language.
Example:

export const Default: Story = {
    render: () => <KcPageStory />
};

export const French: Story = {
    render: () => <KcPageStory 
        kcContext={{
            locale: {
                currentLanguageTag: "fr"
            }
        }}
    />
};

export const Spanish: Story = {
    render: () => <KcPageStory 
        kcContext={{
            locale: {
                currentLanguageTag: "es"
            }
        }}
    />
};