Translation:I will sing a song today.
It isn't a question of what is included in the source language, but what is the correct translation for that. For example, Duo tries very hard to present the natural Japanese by using the sentences with omitted pronouns like "I" and "me". According to your reason, since it is omitted, the translation should omit it also and vice versa. Which is obviously not true, since each language has its own structure and can't be naturally translated word for word. In English the equally correct and a more natural way to say "to sing a song" is "to sing".