cleanup i18n language codes

[luclu]

Rationale:
Stick to language code standards to:

• prevent broken subsystems/incompatibility
• prevent consistency throughout different components (Mist, Dapps, etc.)
• provide clear guidelines for translators

---

Proposal:
Current most accepted code standard is RFC 4646: language in lowercase (ISO 639-1),
then a hyphen, then the region in uppercase (ISO 3166-1 alpha-2). (ref: Camilo Sperberg, 2013, i18n*:Things to take into account)

List of (RFC 4646) language codes, excerpt:

af      Afrikaans
af-ZA   Afrikaans (South Africa)
ar      Arabic
ar-AE   Arabic (U.A.E.)
ar-BH   Arabic (Bahrain)
ar-DZ   Arabic (Algeria)
ar-EG   Arabic (Egypt)
[...]

[luclu]

Currently supported languages:

language	current code	RFC4646 code	change
Albanian	al	sq	yes
Chinese	zh	zh
Chinese (Taiwan)	zh-TW	zh-TW
Dutch	nl	nl
English	en	en
Farsi	fa	fa
French	fr	fr
German	de	de
Italian	it	it
Japanese	ja	ja
Korean	ko	ko
Korean	kr		yes
Norwegian	nb	nb
Portuguese	pt	pt
Spanish	es	es

[luclu]

done in #910

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