Many to Many Table with multiple same-column references
I think unless you have to save space or share a lot of translations you could just do one-to-many mapping documents -> document_translations

CREATE TABLE document_translations
(
    document_id BIGINT NOT NULL,
    language_id INT NOT NULL,
    title TEXT,
    summary TEXT,
    CONSTRAINT document_translations_pk PRIMARY KEY (language_id, document_id)
);


Then you could use external translation memory to help with translations.

If you want to share translations then I don't think you need translated_documents_translations as many-to-many table. Just use translated_documents like this:

CREATE TABLE translated_documents
(
  document_id BIGINT NOT NULL
    CONSTRAINT translated_documents_pkey
    PRIMARY KEY
    CONSTRAINT translated_documents_documents_id_fk
    REFERENCES documents,
  language_id INTEGER,
  title UUID,
  summary UUID,
  CONSTRAINT translated_documents_title_fk
  FOREIGN KEY (title, language_id) REFERENCES translations,
  CONSTRAINT translated_documents_summary_fk
  FOREIGN KEY (summary, language_id) REFERENCES translations
);

CREATE TABLE translations
(
  content_hash UUID NOT NULL,
  language_id INTEGER NOT NULL,
  translation TEXT,
  CONSTRAINT translations_content_hash_language_id_pk
  PRIMARY KEY (content_hash, language_id)
);


And JOIN documents -> documents_translations -> translations