Some parts of the language files have comments indicated by double slashes (//). Do not translate them - they do not show up anywhere in the game, they are there to assist you.

Do not translate the phrase "Mystic's Items" if it shows up anywhere in this file - it's the official name of the mod

Using a JSON text editor is recommended. That kind of software will tell you if there are any formatting errors in your file (for example, a missing quotation mark). If it marks comments as errors, ignore that.

If you want to add a quotation mark inside of your string, prefix it with a backslash (\). Example:
"ITEM_MYSTICSITEMS_TEXTEXAMPLE": "Very \"cool\" text"

Line breaks are indicated by \r\n (or just \n if I forgot to add the \r somewhere)

Do not translate formatting tags that look like this: {ChanceBonus}, {0}, {1}, {ArmorAdd}. They get auto-replaced with numbers by the game.

Do not translate style tag parameters, but translate things inside the tags.
For example, if you see "<style=cIsHealing>increases health</style>", translate only the "increases health", don't translate "<style=cIsHealing>" and "</style>"

"for {Time}s" usually means "for {Time} seconds"
Same with "{Radius}m" meaning "{Radius} meters"

If you want to check what your translation looks like in-game, use the CheatUnlocks mod to unlock logbook entries for all items

Remember: you have all the freedom in your translation!
You don't have to translate wordplays accurately word-to-word if they don't work in your language - you can substitute them with similar puns that would fit better.
No need to translate item names accurately if they're too long in your language.