- Is there some made solution to integrate existing shopping cart values into mobile cart purchase?
A web search says yes. mobi-cart is one of the top ones that comes up - there are others. I can't reccommend anything
- Do you prefer Apple Apps or Android as you posted you currently develope such Apps.
What I prefer is irrelevant. Android just surpassed iPhone in market share in the US, but Apple users tend to be more willing to spend money.
If you covered those two, you;'d have a good chunk of users.
- What language should be used to develope mobile websites from existing ecommerce website in modern way?
There are many languages ways:
I don't have the experience to tell you exactly what to use. Find a shopping cart software that meets your needs, and add mobile capability to the musts list.
For example, I will probably be posting to the ecommerce forum because I'm looking for a shopping cart solution myself. I want it to have: Digital downloads of large files (100MB - 1 GB). Possibly order the same digital downloads on DVD. Sort offerings by state, region, activity, and tags. Be SEO friendly. But in addition: Should have capability to display a mobile version (when the user agent requires it) and look awesome. Should be compatible with some of these mobile shopping cart apps or app generators.
Amazon has mobile website which is not clear published.
Here's the secret: if you go to amazon on a mobile device, it will look way different. Its loading a different theme or whatever based on the user agent. This is what your site should do too.