Enable doc in Spanish (#16518)

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docs/source/en/model_doc/speech-encoder-decoder.mdx

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+<!--Copyright 2021 The HuggingFace Team. All rights reserved.
+
+Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with
+the License. You may obtain a copy of the License at
+
+http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on
+an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the
+specific language governing permissions and limitations under the License.
+-->
+
+# Speech Encoder Decoder Models
+
+The [`SpeechEncoderDecoderModel`] can be used to initialize a speech-sequence-to-text-sequence model
+with any pretrained speech autoencoding model as the encoder (*e.g.* [Wav2Vec2](wav2vec2), [Hubert](hubert)) and any pretrained autoregressive model as the decoder.
+
+The effectiveness of initializing speech-sequence-to-text-sequence models with pretrained checkpoints for speech
+recognition and speech translation has *e.g.* been shown in [Large-Scale Self- and Semi-Supervised Learning for Speech
+Translation](https://arxiv.org/abs/2104.06678) by Changhan Wang, Anne Wu, Juan Pino, Alexei Baevski, Michael Auli,
+Alexis Conneau.
+
+An example of how to use a [`SpeechEncoderDecoderModel`] for inference can be seen in
+[Speech2Text2](speech_to_text_2).
+
+
+## SpeechEncoderDecoderConfig
+
+[[autodoc]] SpeechEncoderDecoderConfig
+
+## SpeechEncoderDecoderModel
+
+[[autodoc]] SpeechEncoderDecoderModel
+    - forward
+    - from_encoder_decoder_pretrained
+
+## FlaxSpeechEncoderDecoderModel
+
+[[autodoc]] FlaxSpeechEncoderDecoderModel
+    - __call__
+    - from_encoder_decoder_pretrained