As you write, the suggested text will appear in the background of the text line in progress and has a lighter color. Tests used to show that this feature is not perfect, sometimes accepting suggestions that only show one word at the end of the sentence. Perhaps the more you use Smart Compose, the more likely it is to recognize and understand the context of the message.
Smart Compose currently only supports English and Google also warns that Smart Compose is unable to provide answers and it is not always true.
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