Write in whichever language feels easiest to use. It doesn't have to be your first language, and you don't have to use the same one every time.
That might sound too simple if you've been going back and forth about it. But there's no single language that makes an entry better. What matters is whether you can write what you want to say without spending the whole time working out how to say it.
It doesn't have to be your first language
Obviously, your first language is usually the one you’re most comfortable writing in.
Writing about feelings is a bit more personal. Some people feel awkward doing it in their first language. The same sentence in another language feels easier to write, even if the grammar isn't perfect. Others feel they can only explain themselves properly in their first language.
So if you've tried one and it feels uncomfortable, try the other. You may find you prefer different languages for different subjects. Writing about work might feel easier in one, while writing about family feels easier in another.
There's also a much simpler reason to choose: you like writing in that language. That's enough for me. You don't need to work out why you enjoy it before you use it.
Using more than one language is fine
A word might come to you in a different language from the rest of the entry. If it says what you mean, you can keep it. You don't have to stop and translate it just to make the whole page match.
This is especially useful when you're writing down something someone said. The translated version may be correct, but it might lose what was funny or familiar about the original. Keeping their words as they said them can help keep the moment clear.
You might use a few words from another language, or change languages for a whole paragraph. If you already do that when you talk, doing it in your journal may feel easier than trying to keep everything in one language.
Writing to practise a language
Maybe you want to write in a language you don't use much anymore. Or you're learning one and want somewhere to practise. Those are good reasons to choose it, even if it takes a little more effort.
I'd start with a few sentences about your day. When you don't know a word, you can look it up or write it in a language you know well and check it later.
The thing to watch is whether you're still enjoying the writing. Looking up a few words can be interesting. Looking up something in every sentence can get frustrating, especially when you have a lot you want to say.
If that happens, you could keep the practice part short and write the rest in a language you're comfortable with. You can decide how much effort you feel like putting into it that day.
Conclusion
Use the language that makes it easiest to write today. If that changes tomorrow, change with it. You don't have to choose one language for the whole journal.
