All English text that will be visible to users should be put in ```app/src/main/res/values/strings.xml```. Any text that is missing in a translation will fall back to the version in this file. Be aware that anything added to this file will need to be translated, so be very concise with wording and try to add as few things as possible. Look for existing strings to use first. If there is untranslatable text that you don't want to keep as a string constant in a Java class, you can use the string resource file ```app/src/main/res/values/donottranslate.xml```.
Each translation has a single file that contains all of the text. A given locale's file can be found at ```app/src/main/res/values-<languagecode>[_<countrycode>]/strings.xml```. So, it could be ```values-en_US``` or ```values-es_ES```, for example. Specifically, they're the [two-letter ISO 639-1 language code](https://en.wikipedia.org/wiki/List_of_ISO_639-1_codes) and the optional [ISO 3166-1 alpha-2 country code](https://en.wikipedia.org/wiki/ISO_3166-1_alpha-2), which is used for a dialect of that particular country.
If you're starting a translation that doesn't already exist, you can just copy the English ```strings.xml``` to a new ```values``` directory and replace the English text inside each of the ```<string>``` ```</string>``` pairs.
Strings follow XML rules, which means that apostrophes and quotation marks have to be "escaped" with a backslash like: ```shouldn\'t``` and ```\"formidable\"```. Also, formatting is ignored when shown in the application, so things like new lines have to be explicitly expressed with codes like ```\n``` for a new line. See also: [String Resources](https://developer.android.com/guide/topics/resources/string-resource.html#FormattingAndStyling).
This project is in the process of migrating to Kotlin, we prefer new code to be written in Kotlin. We try to follow the [Kotlin Style Guide](https://android.github.io/kotlin-guides/style.html) and make use of the [Kotlin Android Extensions](https://kotlinlang.org/docs/tutorials/android-plugin.html).
Existing code in Java should follow the [Android Style Guide](https://source.android.com/source/code-style), which is what Android uses for their own source code. ```@Nullable``` and ```@NotNull``` annotations are really helpful for Kotlin interoperability.
There are three themes in the app, so any visual changes should be checked with each of them to ensure they look appropriate no matter which theme is selected. Usually, you can use existing color attributes like ```?attr/colorPrimary``` and ```?attr/textColorSecondary```. For icons and drawables, use a white drawable and tint it at runtime using ```ThemeUtils``` and specify an attribute that references different colours depending on the theme.
It may refuse to start the rebase if there's changes that haven't been committed, so make sure you've added and committed everything. If there were changes on master to any of the parts of files you worked on, a conflict will arise when you rebase. [Resolving a merge conflict](https://help.github.com/articles/resolving-a-merge-conflict-using-the-command-line) is a good guide to help with this. After committing the resolution, you can run ```git rebase --continue``` to finish the rebase. If you want to cancel, like if you make some mistake in resolving the conflict, you can always do ```git rebase --abort```.