Honestly, I think there are a number of problems- some are structural and some are geopolitical
Firstly, the international business language being English means that UK kids do not have the same incentive to learn another language as DC from minority language countries, and for most of those, at least one of their second languages is English. If you're (e.g) Danish, English is a no brainer if you want to leave the country.
Secondly, the most useful current languages are all very hard- Arabic, Chinese,. Russian, which means it is a lot of hours to get to even basic proficiency and you basically need immersion or huge dedication.
Thirdly, GCSE doesn't differentiate between native and non-native speakers so there's a (possibly wrong) perception that the grade boundaries can be deceptively high. DC who aren't prioritising languages but need "all 8's and 9's" for competitive Unis will drop what they see as risky options to achieve that.
I basically forced both my DC to do a language at GCSE but at their school only about 50% of kids do - ironically DS is predicted a 9 despite not appearing to be able to order a pizza but there you go, so possibly that's another issue- in making them more accessible to encourage participation the GCSE level is not actually useful for anything - but then that contradicts the perception of high grade barriers so not sure what's right there?
Lack of clear career benefits. Language-specific jobs like translation are being lost to tech and it's hard to predict which specific job your specific language will be helpful for. Yes French is helpful if you end up working with French clients but what if you end up working with Italian ones instead?
Also, all Unis have undergone a shift where STEM course have gained massive popularity over the last few decades at the expense of arts and languages. When I was at school there was a national push to get more kids to do maths and science A- levels. Engineering was a relatively low tariff course at most Unis. That has totally turned on its head now.