docs(readme): add machine-translation disclaimer to 7 less-confident translations

For ar, ca, cs, fi, th, uk, vi the AI-generated translation quality
is hardest for me to vouch for. Added a clear "machine-translated
draft — native-speaker PRs welcome" notice at the top of each so
the community can fix them. The note appears below the language
selector for visibility.

docs/translations/README.cs.md

@@ -20,6 +20,9 @@
   🇹🇼 <a href="README.zh-TW.md">繁體中文</a>
 </p>
 
+> ⚠️ Tento překlad je strojový koncept. Revize od rodilých mluvčích jsou vítány formou pull requestu.
+
+
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