Display a guide for installing languages when browser's language isn't en-GB

[Leilei332]

See this comment: https://talk.tiddlywiki.org/t/language-barrier/13317/6?u=xlbilly

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Confirmed: Leilei332 has already signed the Contributor License Agreement (see contributing.md)

[pmario]

I think the problem here is, that my browser language does not match my native language. I use an English OS and browser setting, but I still may want to download a German enmpty edition.

I think the browser settings can be a starting point and it may fit for 80% of the users, but there needs to be a selector.

"Smart" software can make a suggestion based on some heuristics, but the final decision has to made by the user.

[Leilei332]

This PR now uses a language selector button, a browser startup action is created to change the selected language to browser language settings.

[Jermolene]

Thanks @Leilei332 those are good improvements. The approach still rests on the accuracy of the users browser language setting. It would be useful to have a better idea of how many users like @pmario have their browser language not set to their native language. I would guess it would be a small minority of <10%, but I'd prefer to have some data.

[pmario]

It would be useful to have a better idea of how many users like @pmario have their browser language not set to their native language. I would guess it would be a small minority of <10%, but I'd prefer to have some data.

@Jermolene ... I think defaulting the selection to the browser defaults at startup is OK. Even for users like me.

I do have some custom language settings for a reason. So I do expect inconsistent behaviour, depending on the program I start. Eg: My office software is German. The OS and browser is set to English.

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@Leilei332 -- If en-GB is selected there should also be some text eg: "English (British) is the default setting here. So click the "Download" button above.

Or something similar.

May be we should also link to the Full Edition tiddler, that should provide some more info about all possible languages.

[pmario]

Or may be we add the "English - British" info directly to the text before the Download button

[Leilei332]

This PR now displays links to guides for creating new translations if browser's language is not supported by tiddlywiki.