Downgrade untranslatable_diagnostic to allow

See <#132181> for more context.

compiler/rustc_lint/src/internal.rs

@@ -427,7 +427,7 @@ declare_tool_lint! {
     /// More details on translatable diagnostics can be found
     /// [here](https://rustc-dev-guide.rust-lang.org/diagnostics/translation.html).
     pub rustc::UNTRANSLATABLE_DIAGNOSTIC,
-    Deny,
+    Allow,
     "prevent creation of diagnostics which cannot be translated",
     report_in_external_macro: true
 }