multi language support

possible options regarding implementation

	A language specific metadata values "wms_title" "my title" "wms_title" "mein titel"	B language dependent reference data DATA "shapes/rivers_eng.shp" DATA "shapes/german/rivers.shp"
1 key extension	"wms_title_eng" "my title" "wms_title_ger" "mein titel"	---	main question: should resolution happen at i) write time, i.e. only wms_title goes into mapobject or ii) read time, i.e. everything goes into mapobject, extended key e.g. wms_title_ger is used for reading mapfile maintainable? validation?
2 runtime substitution	---	DATA "shapes/rivers_%language%.shp" with language being a request parameter or global setting	already implemented in mapserver for certain parameters all required substitution scenarios (parameters) covered?
3 externalization using external files lang.eng lang.ger	"wms_title" "IDC_TITLE" IDC_TITLE=my title IDC_TITLE=mein titel	DATA "IDC_SHAPES_RIVERS" IDC_SHAPES_RIVERS=shapes/rivers.shp IDC_SHAPES_RIVERS=shapes/german/rivers.shp	common way of internationalization, many tools available to externalize strings at which time the externalized value should be resolved (while parsing mapfile, during msApplySubstitutions, ...)?
4 merge using INCLUDE	---	standard.map LAYER ... NAME "rivers" INCLUDE "rivers.map" ... eng/rivers.map DATA "shapes/rivers_eng.shp" ger/rivers.map DATA "shapes/german/rivers.shp"	mostly implemented in mapserver based on current languange the corresponding map file needs to be included only usable for whole "blocks" (like a LAYER or the WEB/METADATA section)
5 override override certain values of the original mapfile e.g. with override.ger	"wms_title" "my title" #1 1="mein titel"	LAYER ... NAME "rivers" DATA "shapes/rivers_eng.shp" #2 ... 2="shapes/german/rivers.shp"	identification of "key" e.g. by comment like #1 and #2 other option: use XPath-like syntax e.g. WEB/METADATA/wms_title="mein titel" LAYER[rivers]/DATA="shapes/german/rivers.shp"

the already existing ​patch #3608 uses A1 (applied at read time) and B2