A similar question is answered on stackoverflow here. I have just used Android Resource Translator (ART), freeware mentioned by "K G" above, under Windows 7 Pro. For Windows users wanting specifically to translate Android resource files (as originally asked), this is the best free solution I've found. (I confirm that Google Translate is paid, find that memoQ mentioned in a link above is a commercial tool).
ART installed quickly and automatically. I selected output languages by checkbox and it automatically translated my Android app's English res/values/strings.xml file into German, Russian, and French XML files, output under a common "res" directory as values-de/strings.xml, values-ru/strings.xml, and values-fr/strings.xml in under a minute. I then copied these directories under my project's "app/src/main/res" folder and my app has working translations. (I played it safe; you can probably just specify your app's "res" directory for output and skip the copying).
ART uses Bing Translate, and there is no fee involved. This is pure machine translation, so the result may not be as accurate as human translation, but for Android resource entries like "Download" or "Quit" accuracy seems adequate.
Warning: the GUI is unpolished. For example:
- The Windows menu and taskbar icon are titled Android Resource Handler but the application window's title is Android Resource Translator
- I had to stretch the window on my screen to reveal its Translate button
- There is a field containing the text Copy Paste your output folder path. res folder will be created here. Unless you manually enter a directory in this field, I'm not sure any output is generated, or where.
But these don't affect function, only convenience.
Caveats Emptor:
- French translation does not escape apostrophes and this causes a fatal build exception. Workaround: edit French xml file manually replacing ' with \'.
Once I performed translations, I used Android Studio's builtin Translations Editor to validate them.
As K G says, available on SourceForge. Last updated 2015-02-17, 11 downloads this week. Source code available at github. This is "begware;" the author includes a Please donate link in the app leading to a Paypal page.