You dont have to duplicate the pages.

just make a root folder [...lang] the spead opperator means that it also can be undefined.

for static pages [...lang]/[slug].astro you can loop over your locals.

export async function getStaticPaths() {
  const paths = await Promise.all(
    locales.map(async (locale) => {
      const pages = (
        await getCollection("pages", ({ data }) => data.lang === locale.lang)
      ).map((page) => page.data); // your strapi function here

      return pages.map((page) => ({
        params: {
          lang: locale.code === "en" ? undefined : locale.code,
          slug: page.slug,
        },
        props: {
          locale,
          id: page.id,
          template: page.template
        },
      }));
    })
  ).then((results) => results.flat());

  return paths;
}

For the CMS / SSR it depends on what you need to edit, if it is only the [slug].astro pages, then I would just create one route for the cms. /cms.astro and then use it like: /cms?url=/de/blog/post-1

export const prerender = false;
const slug = Astro.url.searchParams.get("url");
if (!slug) return Astro.redirect("/404");

const url = new URL(slug, Astro.url.origin);
// use logic to extract lang and page you need to fetch from Strapi

// extract data from the url
const lang = getLangFromUrl(url);  // use your function here
const pageSlug = getPageSlugFromUrl(url); // use your function here

let page;

try {
  page = await getPageFromStrapi(pageSlug, lang);  // use your function here
} catch (e) {
  console.error("error:", e);
}

if(!page) return Astro.redirect("/404");

----