How to change a wiki page content language?

There is the "move article"-function at the top that allows you to alter the name. I guess in your case you would like to move it to "Tr:Istanbul". Please take care, that you pick an international (here:english) base name for the article.

If you add the text {{languages}} at the source of the article, the page gets an nice translation menu. Further tricks can be found here: http://wiki.openstreetmap.org/wiki/Wiki_Translations

On That wiki, there's no support for multiple wikis per language. Interlanguage links actually use only prefixes for language codes, using lowercase. But for legacy reasons, 7 languages have been created with dedicated namespaces for their language codes that have been capitalized: DE, ES, FR, IT, JA, NL, RU

For all other languages, there's no dedicated namespaces and the prefixes are lowercased.

However because the initial of the prefix is in fact the initial of the pagename, and that wiki capitalizes the first letter of all pagenames, the first letter of the language code (and only that letter) is capitalized automatically. So you'll use "Tr:" for Turkish (not "tr:", not "TR:").

You'll also use "Zh-hans:" for simplified Chinese, "Zh-hant:" for traditional Chinese, "Pt:" for standard Portuguese, "Pt-br:" for Brasilian Portuguese.

Note that variants of the 7 languages are also using lowercase with first letter uppercased.

The supported language codes can be in those forms:

• "LL:" (only for the 7 languages above)
• "Ll:" (e.g. "Tr" for Turkish)
• "Lll:" (e.g. "Vec" for Venitian): note that some 3-letters prefixes such as "Way:" or "Tag:" are not usable as language codes (they are detected as English; they were not created as MediaWiki namespaces)
• "Ll-rr:" (e.g. "En-gb:" for British English)
• "Lll-rr:"
• "Ll-ssss:" (e.g. "Zh-hans")

where "L" or "l" are place holders for a letter of the main language code, "rr" is a country/region code, "ssss" is a standard script code such as "latn", "cyrl", "hans", "hant".

You should NEVER use any code for English, and not use country/region codes (except in "-rr" suffixes), e.g.

• don't use "JP" or "Jp" or "jp": use only "JA" for Japanese
• don't use "GE" or "Ge" or "ge": use only "Ka" for Georgian

Contents for specific countries should not use the namespace-like prefix with colon, but should preferably name the country itself, or use the code elsewhere: those pages will remain translatable and navigatable across languages.

It would have been better if all languages used the same convention for capitalization, but those 7 languages are already detected and managed in the languages navigation bar and lot of templates. The project to use the same lowercase code with a capitalized initial was proposed but never implemented: it would not just require renaming the 7 namespaces but changing many links (and reversing lots of redirects).

So please the convention and avoid creating new unneeded redirects that are hard to fix later when pages need to be moved/renamed: this causes extra maintenance.

Look at the navigation bar: you can unhide the list of languages with missing translations: all red links displayed use the correct capîtalization: follow these red links to create a translated article (after that you may rename your article in your language, but please keep the prefix unchanged.

There's no restriction in BCP47 codes you can use as suffixes (after a colon) for naming OSM tags such as "name", but they follow the standard BCP47 convention (lowercase only for language codes, uppercase for country/region codes in country-specific tags).