To my knowledge, BG journals are always written in first-person plural (We...). In case of those players who are playing with a solo character, the journal isn't being written correctly. I know that this is really just nitpicking.

Using first-person singular (I...) might sound a tempting option, because the protagonist writes the journal, and it seems to be okay in both cases: if he has a party, he still writes in first-person singular, and if he is alone, it's also correct.

Same with encounters. A villain can say: "To arms, my fellows! Let's show them how half-ogres fight!". Or a wanderer/soldier/anyone can say: "Greetings, adventurers!"
These are interesting if the player is playing with a solo character. (But obviously the only way for the developers to avoid this would have been making 'neutral' text /To arms, my fellows! Let's show how half-ogres fight!/)

What's the practice in this respect in mods? I haven't played too many such mods, so I don't know. I'm pretty sure that every mod maker sticks with plural, but I want to hear confirmation. And your opinions about this, of course.

In journals I tend to write "I", I thought that's the style (at least it's in the German version.)

In dialogue, if I want to be correct, I use a trigger checking for other party members to use the proper form, or I try to avoid the form like you mentioned.

Assuming that the PC is not alone but in a group or at least with one other person I only do if I'm too lazy.

I forgot to tell I was talking about BG1, by the way.

I think I was wrong about the journals, probably first-person singular is used there always.
But in case of game dialogs, I was right. There are a lot of dialogue options using plural form. Examples:
This man is named Samuel. We were told to bring him to you.
We had nothing to do with it, we were framed.
Oh yeah, Dabron. Sure we were.
We killed all the dopplegangers, but we weren't able to save Jhasso.
...and hundreds of others.

So I suppose it looks like this:
First-person singular is used for journal entries, while in the dialogues of the original game, the partner (NPC) uses plural, and the player's dialogue reply choices are also in plural.

Eventually I think I'll use singular in my upcoming mod, though.

Thanks for your post, jastey!