Damien,
First, in LabView Help, about "Import VI Strings Method", at the end appears: " Settable when the VI is running NO ", so here is the explanation for your error message...
Anyway, recently I had to make something similar, and searching here i found a couple of ideas to perform this task... At first the solution using subVIs, seemed good to me, but it had some issues and complications... finally i implemented the solution using a very simple structure that it had better performance to do this. Consists in change the "captions" of the controls or indicators and change directly the text of the strings. To implement this quickly, i recommend to use "property node" of the variables and a simple event to detect the "value change" of the "Language control Butt
on". Also you can define several idioms by changing a list. The list can contain the words separated by bar spaces (for example). Resuming, the idea is to create a list with the words (which can be read from a file...), a button control Language, a event to handled de property nodes of the variables and displays. there are a few others details to improve this idea, and i know that there are others ways, but this works perfect and it is easy to implement.
Hope this help.
Regards,
JFT