To explain further, if you were hiring a chauffer for your car, would you choose a newly qualified driver, who passed their test yesterday and is therefore legally qualified to drive your car, or would you look for someone with a license AND lots of experience driving cars over several years beyond their training?
That's a fair point though it can be argued that the most important duties of a flight attendant, the safety aspects, are something that can be taught in a very short period of time (like a couple of weeks) and those skills will probably not improve throughout their entire career, after all, an emergency happens so seldom that their skills in that area are largely untested.
The other part of their job is based around their customer service skills and this is something that will improve as they become more experienced, especially if their employers invest in appropriate training, as all good employers do. Hence, a FA with 15 years experience is a valuble employee, likely to be experienced in all areas of customer service. The catch for the employer is that, firstly, they have to pay them considerably more than a 20 year old with no experience and they have to fund their training.
Of course this is another area where RYR are able to cut costs, they get FAs to pay for their own training and target those with little or no experience or those living in a low cost market, like eastern Europe. This means they end up with staff in their early twenties normally staying less than two years, this appears to be a purposeful policy because it's cheap. Often, this is reflected in the quality of their services.
For the question of English, yes, if they are serving the UK market it would seem prudent commercial sense to ensure their English is fluent, after all, the Brits don't exactly lead the world in their knowledge of other languages, nor are they particularly tolerant of someone who speaks less than perfect English.