If I want a new store on my site for Spain, then I create a new store. I will then be running a multi store version of Magento...no problem.
But if the second store is in Spanish, do I need the language pack? As I don't want automatic translations, as I will be insert correct Spanish for each CMS page/Product page etc.
Or is it that I install the language pack just makes it so default labels within the site are in Spanish? So 'Compare Products' becomes 'Comparar Products' ? And 'My Cart' becomes ' Mi Carrito' etcetc?
Language pack just replaces system labels. It doesn't translate anything. Also themes usually have their own translations, that has to be added separately. Everything even remotely content related (cms blocks / pages / categories / products / configuration and so on) has to be translated separately.
Yes you need to install language pack. Please go through below link it will help you how to setup multi language in Magento:
As already stated the language packs change the core content and 80% of the theme, the rest of the theme you have to do manually. Our partners provided us with the figure of $0.50 per product per translation language, so if you have 5 languages and 5,000 products you are talking about $10,000 in translation costs, 4 x 5,000 x $0.50. Forget about auto translation, you will never get any conversions and it will damage your sites reputation. If you cannot do it correctly it is best not to do it at all or leave in English, you will get a few conversions, not as many as the translated content. but more than auto translation. The auto translation is useful to provide translators with some content to work from as a basis.
You then have another problem which is ranking the translated site separately from the core site, that's not as simple as you have SEO conflicts depending on which method you use which can affect your Google rankings. Then to add even further you have currency conversion issues if not the same currency, every night you need to reload the complete product dataset with the new converted prices from the base currency. Most people think they will double their sales but it is marginal gains, if each translated store added 20% extra it is a good start, but you need customer service in that language. It becomes an all or nothing situation.
Yes, you're right. Basically you need language packs to translate common phrases, labels, and buttons. Language packs don't translate the content of your Magento 2 pages.
If you don't want them to be translated automatically you'd definitely need to translate them manually.
In case you want to even change the translation of labels provided by language packs then I can recommend Magento 2 Translation Extension, and in order to avoid content duplication issues and let Google know what page to direct your customers it's good to use Magento 2 Hreflang extension and connect your pages with the alternate tags.