A lot of it really depends on whether the dev will be working with translators directly or not. Most issues can be overcome pretty easily if the dev is working with the translator.
But here are a few things in no particular order:
1. Avoid using excessive numbers of plugins/scripts that display text. Not only is it a pain to track them all down sometimes, they can easily end up breaking the game when translated (whether due to a minor typo or due to the way scripts sometimes look for an exact term and can't find it if it gets translated).
2. Avoid using images (or at least large amounts of them) to display text. Especially highly stylized fonts, they can be hard to interpret even if one is somewhat fluent and not relying on OCR. Also can be an issue for unofficial translations as it can complicate the patching process if images are encrypted or in an archive (see below).
3. Avoid packing the game into archive(s). This just makes additional steps for the translator and/or end user, making it less likely foreign audiences will play your game (whether it's due to lack of understanding how to patch it or just not wanting to bother with the inconvenience). Obviously if working directly with translators this is not an issue as there would be an official pre-patched version.
4. On that note, avoid aggressive DRM. If the game's data is too hard to get at, it limits the number of people who would be willing to translate it, and the number of end users who would be willing to bother with the patching process. But as above, if the translator and dev are working together, this is not as much of an issue.
Might also be worth noting with those last 2, they also encourage people to just upload the full game pre-patched instead of supporting the dev. Especially the DRM, it never works and just makes people more likely to do the very thing the dev is trying to prevent.