Speak, a startup using artificial intelligence to help people learn languages, is now worth $1 billion in a new funding round, doubling its valuation from six months ago.The company is set to announce Tuesday that it has raised $78 million led by venture capital firm Accel. Existing investors including the OpenAI Startup Fund, Khosla Ventures and Y Combinator also participated. Speak has raised $162 million to date, the company said....