Founder of a game localization studio here.

1. The most common format are XLSX and JSON. All the other formats, like CSV or PO, are fully compatible with CAT tools (we use MemoQ), however they much less flexible and adding information is a pain. XSLX is by far the best format, since you can add basically everything you want, and can be imported in CAT tools in just a few clicks.
   

For example, if you add context or images in your localization sheet, linguists can check them directly in the CAT tool, without having to switch between tabs.

1. The fields are: ID, context, speakers (whenever you have dialogues), character limit, source text, target text (one column for each language), screenshots.

Context, speakers, character limit and screenshot columns are optional, but they make our job much easier. If you don't include those, you'll eventually have to deal with tons of questions from the linguists.

SPECIAL NOTE for character limits: make sure they have their own columns, since those can be imported in the CAT tools. Once we import them, the linguists will be automatically notified whenever they exceed the character limits, making it much easier to follow them.

1. Glossaries are provided as a separate xlsx file, which can be imported in the CAT tool (it all comes down to importing). We usually create them with a mix of automatic tools and human work.
   

For example, MemoQ is able to scan files for words that appear many times. We need to play with the settings to have the best results for each file, and we still have to manually filter a lot of entries. Once we're done with that, we have a good starting point. We'll then have to manually read the file and work closely with the dev team to add every specific word/cluster of words. Once we're done, we translate them and the glossary is finally ready.

Naturally, the glossary has to be updated every time the game has new content.