Spia,
Happy you managed to change the text you wanted to - progress
My guess is 60 to 80% of users start using X-Cart with only basic html (and maybe CSS) understanding or no programming knowledge (so it can be used by non-programmers for sure).
Adding text in smarty is easy (and is not coding based to add the text value or variable) - as Balinor says - read the manual on this (basically at the bottom of the languages page in X-Cart Admin, you just add in a variable, using a variable name which has not been used before ).
To know what has not been used you could export a language and import it into MS Excel (see Internationalization section for importing and exporting a language e.g. Russian language post answer I provided in there) or reading the manual.
The 2nd step is to add the newly created language variable, into a template (or change an existing variable named in a template to match the new one you have just made) is coding based.
At the start you are better off to change the text in a box for existing language variables (rather than add too many new ones) - slowly learn.
Your question... "Is there a big learning curve" - there is some if you have never used html before - yes (but you are 80% there understanding how to use webmaster mode).
Things are easier if you understand basic html programming and then change the syntax a little for it to be php - what I would do initially if I was you is follow mods others in the forum have done (you will then begin to understand how php code works).
For changing text on a page - 95% of the time this can be done within the X-Cart languages section of X-Cart Admin.
If you do not understand basic smarty code (used mainly for languages) or php code then changing things can take time or be hard (if you do not understand basic html first) - yes... there is a learning curve.
But it is not really, really difficult really - as php like html does use pretty easy to read logic (unlike many other programming languages) and the layout is based on html table code a lot.
However the advantage is almost anything can be changed (unlike other carts where code is locked).
Lastly - there are some changes which are harder - logical changes to things like cart $ values or shipping (as these are not a simple change to one template - sometimes this can mean a change in several templates and quite complex logical maths code... not impossible - but harder than layout changes and probably better to avoid unless a custom mod had been provided in the forum which another user has said works).
So layout, colours and text is easier (but function is harder).
At the start - the easiest and fastest way to change colour and text is to buy a custom skin - one which is close to how you want the cart to finally appear - see
www.dana7.com or
http://x-cart-customization.com/shop/ (then change this custom template to match what you want).
Remember that many layout changes are also made in the CSS file (once again quiet easy to do once you try) - e.g. text size
For function changes, for now I suggest stick to completed mods available in the forum or bought ones (and learn from doing those first).
Hope comments help - cheers, Asiaplay
PS: trick is to keep a back-up copy of the original before you make a change - so if you mess up the mod instructions (then you can replace the messed up changed template or page with the original you backed up... then at least you have not lost anything, but a little time).