[IDEA] Remove flags in language plugins

[Leilei332]

As is discussed in this comment

A related point is that my understanding is that it is not recommended to use flags to represent languages (see this article for an example of the arguments against the practice). I wonder if we should change to using the ISO language codes in the switch (eg "en-GB")

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[Jermolene]

Thanks @Leilei332 this is definitely worth doing. I've been aware of the issues with using flags for a long time and it is about time we fixed it.

I pushed some updates to get things building and start to figure out the dependencies that need adjustment. I've also added a generic "language" icon for use in the page control button.

I think some more testing is needed but this feels quite close to being mergeable.