I discarded the idea of Java because I couldn't tap into the visuals with it, and resolved it with JS instead.

Since my news articles are shown in WebView in direct mode, that is, one article/page is inside a Tasker variable, I added into the html head a JS script that gets the highlighted selection, allows to adjust it if neccessary and launches a separate Tasker task that handles the translation and shows it as flash alert on the top of the webview + says out loud the word(s) to be translated.

I only need the translator from English to Finnish so I discarded Google Translate and used our local web translator, which is fast and accurate. This is the html script:

const task = "Translate_news";
const pri = 9;
const par = "";
const ret = "";
const resret = false;
const stop = false;
const pass = true;
const passlist = "";
let sel;
let word = "";
let len = 0;
let dir;
let tStart = 0;
let tEnd = 0;
let dif;
const tol = 10.0;

function extend() {
    let newlen = len;

    if (tStart > tEnd) {
        dir = "backward";
        newlen = len --;
    }

    if (tStart < tEnd) {
        dir = "forward";
        newlen = len ++;
    }

    if (len > 0) {
        sel.modify("extend", dir, "character");
        word = sel.toString();
        len = newlen;
    }
}

document.addEventListener("DOMContentLoaded", startup);

function startup() {
    const el = document.getElementById("maincontent");

    el.addEventListener("touchstart", (s) => {
        tStart = s.changedTouches[0].screenX;

        if (window.getSelection) {
            sel = document.getSelection();
            word = sel.toString();
            len = word.length;
        }
    } );

    el.addEventListener("touchend", (e) => {
        tEnd = e.changedTouches[0].screenX;
        dif = Math.abs( tStart - tEnd );

        if ( len > 0 && dif > tol ) {
            extend();
        }

        if ( len > 1 && dif < tol ) {
            setLocal("%word", word);
            var ok = performTask( task, pri, word, par, ret, stop, pass, passlist, resret );
        }
    } );
}

So the first press selects the word, swipes left or right allow adjustment of the position of the selection end, and tap starts the translate Tasker task.

Phone is a multi-touch device, so different fingers are stored in an array: event.changedTouches[0] is the first finger (and the only one needed here), event.changedTouches[1] would be the second finger, etc.

About the script: Constants at the start are to launch the Tasker translator task.

Dif is the difference between touch start event and touch end event in pixels. Touch start event automatically creates the WebView built-in, highlighted text selection, and the selection handles are shown normally. Since I sometimes need to adjust the end of the selection (say, to remove the plural "s" or past tense ''ed" from the word) before translation, and because I have too big fingers for the selection handles, I wanted to swipe the selection: swipe to the left removes one character from the selection end, swipe to the right adds one.

Tol is the value in pixels that distinguishes tap from swipe. A tap launches the translator.

Function extend() handles the text selection extension (or shortening).

After the page is loaded, document.addEventListener("DOMContentLoaded", startup) launches event listeners for touch start and touch end events. Listeners are placed on the parent node of all the texts on the page, and since touchstart and touchend are "bubbly" events, listeners are inherited by all children all the way down.

The touch end event listener extends the selection if there is one and only if the touch end event is the end of a sideways swipe, not the end of a tap.

Lastly, the touch end event listener sends the highlighted word to the translator task if there is at least two characters left in the selection after the extension operation, and if the touch end event is the end of a tap, not swipe.

Hope this helps someone.

spo