cmnLoc

It's easy to use and does a variety of cool tricks:

• It has short (with option for extra-short) syntax
  With a single-variable setup, "Start" can become L("Start") and that's all it takes to get it localized.
• You can keep your "default language" (e.g. English) in the source code if you prefer.
• Has support for template insertions (L("Volume: {0}%", vol))
• Has support for ICU messages and pluralization
  (no more "message(s)" for non-English)
• Auto-generates files for translation from the source code.
  Multiple formats; command-line and web-based tools.
• Loads translations from multiple formats, easy to add your own.
• Crowdin-compatible import/export options for "public" localization.

Check out the introduction / making of post for a deeper dive into the process and motivations!

Links: itch (cheaper) · docs · marketplace (not cheaper)