Sometimes it's useful to see how long your translation is compared to the source text just to make sure it's not too long and won't overflow the button/container/wherever it will be placed. From time to time I find myself copy-pasting the source string into the text field just to compare its length with my translation.
Here is a proof-of-concept I made. In order to test it, click on any string in Special:Translate to open up the editor and run the code in the developer console. This will add a small counter in the right side of the textarea which displays the amount of characters in the translation and the source text. The result is also depicted in the attached image below.
var textfield = document.querySelector(".open .tux-textarea-translation"); var sourcetext = document.querySelector(".open .twelve.columns.sourcemessage"); var counter = document.createElement("span"); counter.setAttribute("id", "charcounter"); document.querySelector(".open .tux-editor-editsummary-block .columns").appendChild(counter); counter.textContent = textfield.value.length + " / " + sourcetext.innerText.length; counter.style.fontStyle = "italic"; counter.style.position = "absolute"; counter.style.zIndex = "100"; counter.style.top = "-21px"; counter.style.right = "11px"; counter.style.fontSize = "9pt"; counter.style.color = "darkgrey"; textfield.setAttribute("oninput", "document.querySelector('#charcounter').textContent = document.querySelector('.open .tux-textarea-translation').value.length + ' / ' + document.querySelector('.open .twelve.columns.sourcemessage').innerText.length");
Note that this is just intended to act as a guide for approximately how long the translation should be - not as a restriction!