To be in a minority of one in this thread here is my pennysworth...
It is completely unrealistic to be expected to train someone to be fluent in a language without them living and breathing that language for a year or so.... even longer if they need to be able to talk technical jargon. If you can afford to do that then great.
Mean while in the real world if half your staff earn more than you as a struggling business owner then you have to be more pragmatic. I would say the original job role is redundant as it is now fundamentally different in the skillset required and realistically training is not going to bridge the gap. I would make that role redundant and advertise the new role. The current incumbent can apply alongside other applicants if they feel they do actually have the skillset... which they don't Though do discuss this with them and give them opportunities to come up with alternative solutions, maybe a job share or some such which actually might suit them and you better.