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This post was written over 2 years ago, so its content may not be completely up-to-date. Please take this into consideration when reading it.
Back in November 2019, I gave a lightning talk at the PHPSW meetup highlighting how to avoid a few of the pitfalls I often see people struggle with when they get started with localising web projects.
You can find the slides here: https://slides.com/liamhammett/avoiding-the-pain-of-localisation