X member(s) is celebrating his/her birthday today.

Why not just remove the gender placement of his/her and say "their"?

X member(s) is/are celebrating their birthday today.

Depends how big your board is, but I guess often it would only be one member.

1 member(s) is/are celebrating their birthday today.

Looks strange to have "member(s)" and "is/are" but then miss on "their", don't you think?

whats the difference its only a bit of text and doesn't actually make a different to how the board functions or the data is interpretated.

If you switched it you would have the grammar cops on your tail.

'Their' is plural, while 'his/her' is singular.

1 member is celebrating his/her birthday
2 members are celebrating their birthday.

If your board had a lot of members switching it to their might be better, but what about everyone else?

He can just change it to what he wants in the language files.

Their is a word that can be used to describe both female and male genders, while also being a singular form. It is not only confined to the plural form.

*edit* I am fullly aware I can change this in the language file.

You are aware you can change the wording in your lang files ?

I don't think it really matters to be honest, if it really gets on your nerves, change the language file.

Their is a word that can be used to describe both female and male genders, while also being a singular form. It is not only confined to the plural form.

*edit* I am fullly aware I can change this in the language file.

It probably would be best left to just changing the language file though, don't you think?

Also, as far as I'm aware (and I am not an English major), their can be used to replace a singular antecedent such as someone or anyone, but not to be used in place of the singular pronouns he/she.