Have I to specify the sentence and paragraph when referring to the law?

You could come across such citation and it would be appropriate.

Translation choices for Ukrainian legal citations depend on the target audience, an inhouse style, and personal preference, so it should be off topic here to discuss the pros and cons of different approaches in detail. For instance, we could follow the citation rules which would be expected in the country of our client or in the place of arbitration. We could mimic the style of other documents of our client. We could translate references literally to ensure that they survive translating them back into Ukrainian. The latter could look like your example in English.

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Speaking of whether you have to reference the sentence, a short answer is that you as the author decide how precise your reference needs to be for the reader to find the rule which you reference. I agree in this with another answer, which is already here.

We usually do not reference sentences because it is not necessary in most cases. The sentences are not enumerated in a text of a law, which also makes referencing them by number inconvenient. But if we want to drive attention of the reader to a rule in a long paragraph or we want to amend only some words in a paragraph, we may reference the sentence.

In Ukraine we would rather reference a sentence with full words like "second sentence" or "last phrase". A few authors use "реч." as a shortening (from Ukrainian "речення") followed by the number indicating its position in the paragraph of a law. This is what you would expect under common abbreviation rules. But it is quite unusual to use that shortening for referencing laws. I am not aware of a corresponding notation in English legal citations.

If, as you say, there is only one sentence in a paragraph, then referencing that sentence gives no additional precision. For comparison, if there is only one paragraph in an article, there might be some additional precision in referencing that paragraph in case any future amendments add new paragraphs. But it is for you to decide whether you want to worry about that.

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The tradition dictated to cite the Criminal Code beginning with the article and progressing to lower elements from top to down. When articles represent specific crimes, it seems intuitively appropriate to start citing with the article when you reference a criminal offence. But nowadays, probably for consistency, the Criminal Code of Ukraine is typically cited the same way as other laws in Ukraine, which is beginning with the lowest element and progressing up. When orienting within a law or an article of this law, this approach puts the changing part of the citation upfront allowing to recognise the rule faster, especially considering that the name of the law is usually cited in full words with indication of the number of the law and the date of its adoption, which is usually important because it is difficult to be sure in Ukraine that there is no other law with similar or the same name now or at some other time. There are usually no globally recognised official abbreviations for the names of the laws.

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Concerning your example in Ukrainian, you refer to the top element of an article as "аб." I assume you mean "абз.", which is the shortening for "абзац". However, the top element of an article is typically "частина" (Russian "часть"). They can be enumerated or not. If they are enumerated, then the next level element would be typically "абзац".

To complicate the situation, there are also laws which adopt their own names for the elements of their articles and even impose their own citation conventions. The Tax Code of Ukraine is an example.

I take the article below from a translation of the Law of Ukraine "On the Regime of Foreign Investments" to illustrate how tricky it is to achieve consistent use of terminology. The highlighted parts are the same reference which is worded exactly the same in Ukrainian. But they were translated differently and different translations for the name of the first element of the article were used, apparently by accident.

Article 10. Compensation and Reimbursement of Losses Incurred by Foreign Investors

Foreign investors have the right to receive compensation of their losses, including lost profit and moral damage incurred as a result of actions, inaction or improper fulfillment by the state bodies of Ukraine or their officials of the responsibilities in relation to foreign investors or entities with foreign investment stipulated by the legislation, according to the legislation of Ukraine.

All expenses and losses incurred by foreign investors as a result of activity described in Article 9 and the first paragraph of this Article, are to be compensated on the basis of current market prices and/or substantiated valuation, certified by an auditor or auditing company.

Compensation paid to the foreign investor shall be prompt, adequate and efficient. Compensation to be paid to the foreign investor as a result of the actions mentioned in Article 9 of this Law shall be calculated as of the moment of the cessation of the property rights.

Compensation to be paid to the foreign investor as a result of the actions mentioned in the part 1 of this Article shall be determined as of the moment of making the decision regarding reimbursement of losses. The sum of compensation shall be paid in the currency in which the investment has been made, or in any other currency acceptable to the foreign investor, according to the legislation of Ukraine. From the moment of receiving entitlement for compensation and up to the moment of its payment, interest for the amount of compensation shall be accrued at the average interest rate, at which London banks provide credits to first-class banks on the European foreign exchange market (London Inter-Bank Offered Rate - LIBOR)).