Admire my confidence - passive aggressive much? I'm not going to get in to a big debate about this as the OP hasn't come back on to comment on any posts since her first (where are you OP?). However, I put my perspective and opinion across, perhaps poorly, so I will attempt to be more specific in what I was getting at.
At 14 to 18, GCSE's are the be all and end all to most children (and parents); they work hard and parents encourage, praise and sometimes reward, in an attempt to steer and guide their child in the right direction in achieving a set of grades which will stand them in good stead.
This then happens again in further education, or if they seek employment, and again at higher education - obviously making important decisions that may affect their career throughout their life so lots of thought goes in to it.
There's a lot of pressure to be taking a 'broad' range of GCSE's and many kids and parents become stressed and loose sight of what the individual (the young person) actually likes doing. Gosh, even some teachers will have their motives when advising what options children should take (they have league tables and targets to meet, they won't want potential 'lower grade' students bringing their GCSE grades down....).
So, my first point being - listen to your child. What are their interests? What do they enjoy? What are they good at? What do they want to be when they're older? Then from there, map out what GCSE's will keep them challenged, interested and give them a start in their career (if they know!) If language is enjoyable and may be beneficial to them, then absolutely pursue it.
My second point is to do with what potential employers look for, and I disagree that a foreign language is an important one. Perhaps I'm naive on this point (I'm a reflector so I will consider this!) but to me, between 16 and 22, a young person has either chosen further or higher education, or ventured in to the world of full time employment. At the age of 22, a foreign language GCSE isn't the be all or end-all unless they've chosen a career path that meant it was necessary to know/speak another language other than English. And if this was the case, they would have pursued this at AS or A levels or at Uni. So, post 22 years old, most employers would be more interested in what they have done between the ages of 16 and 21 in Education, work experience or employment, and GCSE's would of course be reviewed, but not in as much 'depth' as period after GCSE's.
I'm not 'playing down GCSE's but I'm trying to explain that many employers, wouldn't look to see if the person had a foreign language GCSE unless specific to job role. I have worked in large international and national organisations and small ones across a variety of sectors from retail, manufacturing, logistics and finance/insurance and head offices (so marketing, buying, finance, IT depts). In my honest opinion, I've never come across a job role vacancy that required a GCSE in a foreign language, nor has an appointment been made whereby a candidate with an additional language has meant they've bagged the job - it just hasn't come up so I can only base my opinion on experience of working in HR for 10 years.
Perhaps a foreign language could improve overall communication, I have no idea about that point, but in the context of the OP's dilemma, the DD already speaks Welsh, so if true, would have good communication already.