Swedish (modern) & Swedish (traditional) translations

[shoogle]

There was disagreement among Swedish translators over whether to use modern or traditional terms in the Swedish [sv] translation. To resolve this, I've created a new language for the traditional terms, and updated the names of both languages in Preferences to distinguish between them.

Code	Transifex name	Preferences name	Comment
sv	Swedish	Svenska (modern)	Our existing Swedish translation
sv_SE	Swedish (Sweden)	Svenska (traditionell)	The new translation

There isn't a language code for "Swedish (traditional)", hence sv_SE is used instead.

The existing translation was copied and then edited to create the new translation. See second commit for the strings that have changed.

[cbjeukendrup]

We should think about which consequences this has for automatic language selection based on system language code (see LanguagesService::effectiveLanguageCode).
With this PR, users whose system language code returns sv would get the modern variant, and users with sv_SE would get the traditional variant, but we might want to redirect both to the modern variant by default (for example).

[shoogle]

@cbjeukendrup, good point. I've updated the PR to prefer the modern translations when using the system locale. Users would have to choose the traditional ones explicitly, if that's what they want to use.

I confirmed it shows the modern translations on Windows when the system locale is Swedish (Sweden), and that the traditional translations are shown if they are manually selected via Preferences.