If you regularly translate text from or to the same language, you can set default languages. Once you have set the languages, Word will automatically translate the text as soon as you hit Translate, without you having to set the text each time.
To set the default languages, go to Review | Translate | Set Document Translation Language.
Word sets the default languages separately for selections and whole documents, so you can, if you want, set different defaults for each. This means that when you set your default languages, you must click on both the Selection and Document headings in the Translator sidebar, and select the languages for each, even if they are to be the same for both.
Under each heading, simply select the languages you want, and either click Translate if you want to translate the current text, or simply close the sidebar.
Word will remember your language choices, and next time you translate something it will translate it immediately, skipping the step of choosing the language.
NOTE: As Word is generally pretty good at detecting the language you are translating from, you may prefer to leave the From language as Auto-detect. If you have a From language selected, and try to translate from another language, Word will simply leave the text untranslated. Leaving it on Auto-detect gives you the freedom to translate from any language you come across, without having to change the settings.