How can it be fixed?
What were you doing when you got the Yandex error code 410?
I suspect you do not have a 'default language' set when you try to RequestTranslation when the app expects to have a value but does not. (When you ask for Arabic (ar) as the language to translate to). 
Perhaps you cannot translate Arabic to Arabic using your first RequestTranslation Block.
It was working well with many languages i added but suddenly it stopped
yes that error,
How can it be fixed, no issues with the blocks as it was working before with most of languges i added
It was working; it is NOT now.
- you can use the aia file you made before you added more languages because that WORKED.
- this block/blockss might be wrong
- you can remove languages one at time then test the modified code until your logic blocks work. The problem started probably because one of your IF statements is not correct. Which one, I don't know.
- you can make an example of your code that does the language selection and post the aia here so others can see what might be wrong.
Only you can fix the behavior
I am having the same problem: error code 410. The app I used successfully to translate between several languages for quite some time, only works for some languages now and not the ones I need: Dutch and Danish.
I rebuilt my app, to no avail, alas.
XLate25.aia (35.3 KB)
So Yandex, which we have used for years, has shut down its translation service. Because App Inventor runs translations through a proxy, I was able to switch the proxy over to use Amazon's translation service. So apps should work again without modification.
Note: Amazon may have different translations vs. Yandex.