I'm trying to figure out exactly what these php.ini settings do. What happens when they're set to different values? When are they necessary? When are they harmful?

• mbstring.language
• mbstring.http_input
• mbstring.http_output
• mbstring.encoding_translation

As usual, the PHP manual is less than helpful.

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You can change mbstring.language to whatever language you are using with. (Source)

language

; language for internal character representation.
mbstring.language = Neutral ; Set default language to neutral(UTF-8) (default)
mbstring.language = English
mbstring.language = Japanese
mbstring.language = Korean  ;For Korean market later

http_input

; http input encoding.
mbstring.http_input = pass
mbstring.http_input = auto
mbstring.http_input = UTF-8
mbstring.http_input = UTF-8, SJIS, EUC-JP

http_output

; http output encoding. mb_output_handler must be
; registered as output buffer to function
mbstring.http_output = pass
mbstring.http_output = UTF-8

encoding translation

; enable automatic encoding translation accoding to
; mbstring.internal_encoding setting. Input chars are
; converted to internal encoding by setting this to On.
; Note: Do _not_ use automatic encoding translation for
;       portable libs/applications.
mbstring.encoding_translation = On