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I made the correct page Portal:Mrh and prepared the code for its classification.
Still, I did not enable the language (I cannot do it myself).
Note that it is not absolutely required to have at least a Wikipedia incubator. But it should be useful to request one, so that you can start working with a test wiki and then the supprot on this wiki will be to start translating the Wiki user interface and test it in the Wikipedia incubator. But you can still create an incubator and test it with another UI language (English or any other existing supported language of Myanmar or India, notably in Mizoram which also has languages supported and using the Latin script as well). Try convincing other users in your region to join your efforts for this Wikipedia incubator and test the UI: if you're ready with such group, and you provide to the admins of this wiki a test wiki (Wikipedia incubator, or Omegawiki) where it would be desirable to have an UI), your request will be satisfied more easily.
But translating an UI in a language for which there's no wiki content at all any where to use with this UI is unlikely to succeed. But if you succeed, you'll start creating an UI, that will be tested in Wikipedia incubator or Omegawiki, and later if you pass a good enough translation level (at least the 500 core messages for MediaWiki), you'll get a chance to request the addition of another full wiki (but still you'll be able to continue your work on the test wiki, and to improve the support for amny other languages on this wiki).
In all cases, you need to create some content in the requested language on a test wiki before you can request translating the UI and test it. Don't work alone: this can be easily achived with a small group of people. May be a Wikipedia incubator wiki is not the first goal you'll want, and you may choose an incubator Wikitionnary (which is a very interesting thing to do to support your language, and create and document a stable terminology and orthography), or a test incubator for Wikibooks where you'll want to publish some lexicons, or a test incibator for Wikisource if you want to reproduce old free litterary works from some free corpus of texts in your language (ask your local book libraries if you can reproduce them or ask for religious texts, or ask for public documents produced by your local administration in that language).
This is now enabled.
For example, you can start translating MediaWiki to Mara at https://translatewiki.net/wiki/Special:Translate?filter=%21translated&action=translate&language=mrh&group=core-0-mostused
I just removed the "disabled language" notices in the portal and related categories
Why babel user information is in English? How do I change it into Mara?
They are translatable as part of the standard translatable projects for Mediawiki on this wiki. Just translate them the normal way with the standard translator interface. There's a dedicated subgroup for "Babel" strings. what you see is just the standard fallback to English, because these messages are still not translated.
If you want, you may request that this language fallbacks to another language (Burmese? Hindi? or even Bengalese?) before falling back to English. But India and Myanmar have lot of national languages, and English is generally used in both countries as the common "de facto" interchange language between communities speaking different languages.
If another fallback is used for Mara Chin, it could be to another more "popular" regional language (such as Mizo) directly tied with Mara Chin, in the same region of Mizoram in India or in the nearby region of Myanmar.
But translating the Babel box messages is usually not a priority for any project (and not translating them has never been blocking the deployment of any wiki). Most users want first to haver the core Mediawiki UI messages translated because they will be constantly exposed to these messages.
Translating Babel boxes is very secondary and many users do not even use Babelboxes on their user pages (and Babelboxes are not used on all wikis that prefer to use something else (or that do not want to support "user pages" in the "User:" namespace, and just want to support their own community forums, and otherwise use their own system of private mailboxes or interpersonal communication tools, including chats). Babelboxes are also not suitable for general talk pages.