Всё, что он говорит, я понимаю

[Адов]

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Цитата:https://bkrs.info/slovo.php?ch=得
2) глагольный инфикс, выражающий возможность совершения действия
生水喝不得 [shēngshuĭ hēbude] - сырую воду пить нельзя
他走得动 [tā zŏudedòng] - он в состоянии идти
我买得起 [wŏ măideqĭ] - я в состоянии [могу] купить

听懂 я понимаю (по речи) I understand (by speech)
听得懂 я могу понимать (по речи) I can understand (by speech)

всё, что он говорит, я понимаю.
Actually it is just 听懂. But this situation is obviously about ability, so you can also say 听得懂.

[сарма]

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Лёвыди, 他说的话我都听得懂

[Адов]

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他说的话我全都懂
他说的话我全都听懂
他说的话我全都听得懂
他说的话我全都明白

I prefer to use 全都. Of course, I can think of more possible translations, but it is better not to make things complicated.

2017.02.11Oleg2 сарма, 他说的话我每一句都听懂？

每一句 (каждое слово, что он ...) is not the wording in the original sentence. Better not put things into the author's mouth.

[Адов]

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2017.02.11Johny It's absolutely correct. You can use it.

Of course if we are saying correct/wrong 能听得懂 (can + be able to understand) is not wrong. But here it is the case of good/bad (as r1 asks about good grammar). Just like wearing an unfit coat is not wrong, but not good. In our school classes, usually we have 病句改正 exercise, and in many cases we are asked to remove what is superfluous.

[Oleg2]

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сарма, 他说的话我都听懂

ЗЫ Хотя, сказал бы 他说的话我都能听懂.

[Адов]

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2017.02.11r1 Is using 能 with +得懂 considered good grammar? For instance, 能听得懂，能看得懂。I see it used but it's something we're usually taught  not to use.

I will say it is not good grammar, because 能听懂 (can understand) or 听得懂 (can understand) is enough, 能听得懂 (can be able to understand) is superfluous.

2017.02.11r1 Also is there any difference between 能听懂 and 听得懂?

In this case, they are the same.
你能听懂中文嗎﹖and 你听得懂中文嗎？have no difference.
你能看懂中文嗎﹖and 你看得懂中文嗎？have no difference.
But for me if a sentence has мочь, I will surely translate as 能, and if a sentence has 能, I will surely translate as мочь. Otherwise, I may not include the word мочь/能.

I have to stress In this case, because 能(_)懂 may not be in every case = (_)得懂. And very often the meaning is the same, but the feeling is not right, or the habit of the language is not that.

[Opiate]

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2017.02.11Хон 英语是国际语言吗?

Да.
国际朋友 = иностранец
国际语言 = английский язык
Иностранцы говорят на английском языке, логично.