Disclaimer: I'm not part of Mojang and there's nothing official about this. I'm just posting in the hope it will be helpful. It seems similar questions come up again and again and I think it would be good to have a central place to share information. Crowdin does a pretty good job with the day to day info, but general advice is a bit lacking. I'll add to this post as necessary. Mods - if this looks useful please pin!
Check your language's Discussions tab for hints/issues
Say hello to the team - don't be a hit-and-run!
Go back to the Files tab and hit the Translate or Vote button!
I can't find my language!
Send Jeb an e-mail asking him to add it - Jens Bergensten <[email protected]>. Be polite and patient - he probably has one or two things to do.
A bad translation is stuck with lots of votes
It can happen. You can also get different translations of the same word winning different votes. These situations can only be solved by a proofreader. You can contact Jeb to request proofreader status, but I suggest you do it only after discussing it with your language group. A proofreader can approve or delete particular translations.
Avoid machine translation!
Computer translation has come a long way, but it won't give you a quality translation and for many languages it will give you complete rubbish.
If you don't believe me here's my back-translation of that sentence after being translated by Google into Irish:
Computer translation coming upon a long way, but won't give him to you translation of quality and to many languages will give him to you whole rubbish.
It's hard to convey the awkwardness of the translation when putting it back into English because it makes the mistake of just using English structures which don't work properly in Irish.
Here's what Google makes of my translation (i.e. one that makes sense in Irish): Has developed a computer translation but you get high quality translation from and in many cases you will not only nonsense right away.
Use Comments, Terms and Context
The crowdin.net translation interface has a Comment section at the side. Often other language teams will have already put useful advice or suggestions there. Read them!
You can also add definitions of Terms used in any particular string. This can be useful for keeping a consistent vocabulary through the translation. One thing to remember: copy the English description into your language description if you create one; otherwise it won't appear for your language.
The top of the translation area will also show you the Context. Usually this is just the code label (e.g. "Minecraft Realms" context is "menu.online"). This can give you a hint about where to look in the game to see how the words are used in context. If it's very ambiguous or confusing additional comments can be added to the context.
It's very easy with crowdin to check how words have been used before. Just have another translation tab open and use the search feature (the magnifying glass at the top of the "Texts to translate" list) to search for the English word you want to check.
For example, carpet was recently added to Minecraft so you have to translate the various colours of carpet. You can easily search for unusual terms like Magenta or Cyan to see how Magenta Wool or Cyan Wool were translated in the past.
No translation can be better than a bad translation
If you're not sure about a translation, either take the time to find the best solution or just leave it to someone else. We all immediately recognize a bad (or even just lame) translation. If people see too much of that they'll just swap back to English.