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Many many thanks to Hervé who solved this problem. He put in many hours to fix this over and above. It's a measure of his dedication to and knowledge of XOOPS that he eventually tracked it down. A first class bit of work.
I am, of course, the cause of the problem in the first place. I created the CRI language folder so that we at CRI could customise the English on our site—communication.org.au— for our specific needs and users. In doing so we did not realise the programming implications of what we were doing.
For me this raises two important issues for the XOOPS community.
1. Already raised by Hervé, is the fact that the fault was not showing up in the debug mode. I don't know enough about programming to suggest a fix, but it should be possible to do so and should be added into the debug mode.
2. XOOPS is built to be customised: templates, themes, and languages are all modifiable. It's one of the things that makes XOOPS such a good platform. But it is not clear, as in this case, to a non-programmer, or someone unfamiliar with the overall architecture and dependencies of code are, where the limits of that customisation are, when a change is likely to have implications beyond the specific file one is working on. It might be helpful to all users if there were notes in folders and files signalling those limits.
I don't know how practical that is, but it might help people like me, who want to use XOOPS to the full, but would rather not create hidden nightmares for brave and highly competent people like Hervé to sort out.
This is just a suggestion. In the meantime hurray for for Hervé!