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Well, I understand that "Russian" is a stop-word here even though the business is Lithuania-based and is supposed to work across the Baltics. Anyway, I have found a solution myself.
For all those who use Lithuanian, Latvian, Estonian, Ukrainian, Polish, Russian or any other language Squarespace pretends doesn't exist as their main website language, you may change any uneditable hardcoded text with CSS.
You will need to know:
The ID of the section/block that contains the object you need to edit. You can find this using an addon for your browser called Squarespace ID Finder. Available for Chrome and Firefox.
Blocks IDs look like this: #block-a72b4f2f714d79fd041b
  The ID of the object you need to edit within that block. You can get it by turning console on in your browser (usually with the F12 key), switching to the "Pick" tool (Ctrl + Shift + C in Firefox or Chrome), hover over the object to be edited and click it. The ID you need will be displayed by your pointer:

Then the console will jump to and highlight the respective source line for the object:

The part in blue on the top screenshot (.pricing-plan-price-billing-period) by the cursor or the part after the "class" on the bottom screenshot in the console is what you need. Keep in mind that a preceding full stop is obligatory to address an object (see example below).
Now go to Website > Website Tools > Custom CSS, then write this in the editor, changing the IDs to yours:
//You can keep these comments for your convenience. //First you must hide the original text <your Block ID> <your ID of the object with preceding full stop> {visibility:hidden;} //Then you should add your own text using ":before" after the ID <your Block ID> <your ID of the object with preceding full stop>:before { visibility:visible; content:"<Your Text Here>"; } Here's an example:
//I hide the original text which I don't want to be shown anymore #block-a72b4f2f714d79fd041b .pricing-plan-price-billing-period {visibility:hidden;} //Then I add my own text before it instead: #block-a72b4f2f714d79fd041b .pricing-plan-price-billing-period:before { visibility:visible; content:"Vienas mokėjimas"; } Click Save, refresh your page, see that your text is now shown instead.

If you want to apply your changes to this exact class site-wide, don't use the block ID in your CSS code.
E.g. in my case if I delete the block ID, the "One time" line in the price block will be translated on all pages. With the block ID in the code it only applies to one block, since it has unique ID across the website.
I'm sure there's a better way to do it, and maybe some pro volunteers to drop in, but this method works. Its downside is that the original text remains there, albeit invisible, thus occupying some space.