I already explained you why it shouldn't be done (mind the wording!):
- Textareas allow newlines
- Translations are hard to read
- You need your mouse to translate
- Use-cases showed it is a poor solution
- Months ago it has been established this is not the way to go.
Now before I will go into each point giving you more verbose information on the how and why, I first want to tell you something.
I have a day job, and next to it I (for free) work on OpenTTD. Much of my time (and effort) goes into programming, and making cool new features. Although I don't mind questions and suggestions like you made from time to time, I do not have the time to answer all of them in full in the way the user would like to. Believe me, they always want to know more and more. Like you. Not meant in any offensive way, but the list above is not enough for you to leave it at that. From your perspective, I completely agree. Just realise that WT is not designed over night, and a lot of thought has gone into it. The nature of a human defines that he always knows better (mostly in a good way), and so that the author of an application must have missed something obvious. Hell, even I do

Just understand that I get questions like this a lot, and often. It is most often easier for me being rude and saying: no, because I say so, to stop the endless questions, endless "why?", etc etc. So just be aware that such questions consume a lot of time from my side, time I could have spend better (in my opinion), mostly because I do not get paid for this!
- Textareas allow newlines
This is a problem of suggestion. When you see a textarea, you want to press enter. In translations, enters are not allowed. So to imply you can press enter, will give user questions, will result in developers needing to spend time answering. This is unwanted. An inputbox does not give this suggestion. This comes with: a textarea in HTML is meant to have multiline input.
- Translations are hard to read
In textarea it becomes rather hard to read how your translation is doing. This is even worse for RTL languages. Early use-case tests showed it was much harder to translate with a textarea than an textbox.
- You need your mouse to translate
Just a minor user-friendly thingy.
- Use-cases showed it is a poor solution
For the few days I did have a textarea, many many issues were detected and found. It simply didn't work. And this I guess is the most prominent issue: it doesn't work. Not in the structure and design WT3.0 is in.
- Months ago it has been established this is not the way to go.
Meh, nuff said.
I see this thread mostly derailed in it being a technical issue. Although there are a few technical issues, they can always be resolved (over time) and are rarely a point of not adding anything. My points are mostly design-questions. To which the answer comes back every time I got asked (or asked myself): textareas do not work in WT3.0. It needs a design change. Something I planned for WT3.1, but WT3.1 is only in 'planning'. Mostly because I don't have time :'(
Right, I hope this reply clear things up for you a bit. I would like to end with what I already said:
Still, valid suggestion. Just not easily implementable, so postponed
