Separation of content language from UI language
Currently it is impossible to use Redmine with English UI to work on Persian content. Because English is an LTR language and Persian an RTL one. So all text fields and authored content will be shown as LTR to the user. The same problem happens if somebody wants to use the Persian UI of Redmine with English content. For example I cannot set my locale to Persian on this site, because if I do so, all the English messages (issue description, user comments, wiki pages, ...) and the text boxes will be shown right-to-left to me, while they should be shown as left-to-right.
A simple part of solution would be letting the manager of a project set language of contents of the project. (The site-wide default locale would be a good default for project content's locale.) Then all the text fields in that project must obey direction of that language instead of user's UI locale.
A more advanced solution would include multi-language features. For example a wiki in which the language is settable per page. This way one would be able to translate some pages of his wiki to different languages. Another example would be letting the manager translate project description to different languages to show the appropriate one to each user based on his UI locale.
There is a workaround until this feature is fully implemented: To assume that language of all content of a site is same as language of the site-wide default locale. For example if default locale of site is English, all text fields and messages are shown as LTR even in RTL locales, and if the default locale of site is Persian, then all of them are shown as RTL even in LTR locales. This way I can use Persian interface of this site as soon as possible! :D