General questions
Q: I want my language to be added to the game. How can I help?
1. If your language doesn't appear in the game list at all, you first need to ask the developers to add the language in Crowdin. Do so here: Web based translations
2. It's now up to you to help translating the game. It won't be fun if you try to do it all by yourself, ask your friends to join in!
Q: How can I contribute to my language's translation?
A: The translation of the game is made through the community website Crowdin. Join Factorio's project! It is quite user-friendly: suggest a translation and vote for other people's suggestions. The translation with the highest score will be updated in the game in the next release.
Q: I want to report a mistake in the English text, but Crowdin doesn't support English translation. What can I do?
A: Find the related string in Crowdin. In the comment section, report an issue in the comment section. Choose the type "mistake in the source string" and suggest a better phrasing. Your proposition will be reviewed by the developers.
Q: Factorio translation sucks, English FTW!
Factorio is voluntarily translated by the players, therefore they are not professional. This means:
a. you don't have to bash people that only tried to contribute to make the game better...
b. sometimes a strong fan-based translation can be better than a not-so-invested professional one...
c. they can use your knowledge and feedback by joining them and suggesting better terms in Crowdin. You don't have to spend a lot of time into it, just create a new topic in your language's forum and explain your opinion.
Crowdin-related questions
Q: There are several ways to translate this string, it depends on the context. How can I know which of the translations to use?
A: Below the English text, you can display the context of the string. Sometimes the "technical" name is enough, in other cases you want the developers to tell you how this string is used. Then you need to raise an issue in the comment section. Choose the type "lack of contextual information" and explain the different interpretations you have of the string so that the developers understand the ambiguity and update the context.
Q: A text is not translated in the game but a) it's already translated in Crowdin, or b) I can't find it.
A:
a) The release process makes it so that new texts introduced by a new version do not always appear on Crowdin before its release. There is a high chance that the text was translated after the release, you'll have to wait for the next version to be released.
b) Make sure in your search that you don't have undesirable filters, especially based on the file. If you definitely can't find the string, you can report it as a bug in the appropriate section.
Q: A word-to-word translation of this string doesn't make sense in my language. Do you mind if I try something more creative?
A: It is completely normal that some strings are harder to translate into a language: that's the difference between translation and localisation. It is even harder when there are cultural references (in particular the achievements' titles). In such cases, don't hesitate to fit to your culture and way of saying, as long as it sticks to the original meaning/reference.
Q: Because of the way Crowdin works, sometimes a single term is translated differently across multiple strings. How can we prevent that?
A: I suggest that you first settle on the translation of the single term: use Crowdin's forums to start discussing about it. Once you all agree, update all related strings so that only the phrasing of the string gets voted up/down, without the term changing.
Q: What's the difference between "translated" and "approved" (or validated)?
A: Anyone can join the project and suggest translations. The one with the highest score will be included into the game. There is however a way to bypass these rules: some contributors have been granted the rank of "proofreader". It is the equivalent of a moderator: they can remove or approve suggestions. When a suggestion is approved, votes and suggestions are not taken into account anymore: the approved suggestion will be used no matter what. This is a way to set a translation in stone so that other strings that relate to this one are consistent with each other. Approving translations is not mandatory.
Feel free to contribute to this list or ask any question, I'll update this post once in a while.
Note: As you can guess I'm invested in the translation project because I'm not a native English speaker. This might explain some typos I made in this FAQ: sorry about that!
I will happily let any grammar nazi moderator edit my post to correct them. 
