Assalaamu alaykum wa rahmatullaahi wa barakaatuhu. Shaykh, may Allah reward you for your previous answers. Shaykh, I had two queries:
1. Though I read on your website that standing for and singing the national anthem of one’s country is forbidden, my friends are asking me about the reason for that. They are saying that it is not worship; you are just showing your love and respect towards your country. They claim that our Prophet, sallallaahu 'alayhi wa sallam, also loved Al-Madeenah. I said to them that he did not stand and sing like we do and that this is part of Western culture. My friends say that it is all about one’s intentions and that they are not worshiping by doing that. How can I make them understand in light of the Quran and authentic hadiths?
2. Is it forbidden to wear football jerseys of foreign players such as Messi and Ronaldo? Like, if one has these players' names and their respective jerseys number one the back of one’s shirt. One of my friends says that one is thus promoting disbelievers? Does it constitute promoting them, or one can wear it?
Please take your time and provide me with authentic evidence. Please do not just refer me to a similar, related question. May Allah reward you.

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger. 

We are forbidden in the Shariah to imitate the non-Muslims and it is dispraised to follow their ways. This prohibition and dispraise will not become permissible by having a good intention, so whoever imitates them and follows their way has committed what is forbidden, even if he has a good intention.

Do you not see that the Shariah forbade us from praying at certain times because the polytheists prostrate at this particular time to the sun.

The Prophet said:

"Pray the Fajr prayer and then stop praying until the sun rises and is high up in the sky, as when it rises, it rises between the two horns of the Devil, and at this time the disbelievers prostrate to it. Then offer the prayer, because the prayer is witnessed and recorded till the shadow of a lance becomes equal to it in length. Then stop praying, because at that time Hellfire is heated up to its utmost. When the sun passes its meridian, pray, for the prayer is witnessed till you pray the ‘Asr prayer; then stop praying until the sun sets, for it sets between two horns of the devil, and (at that time) the disbelievers prostrate to it." [Muslim]

The Muslim is forbidden from praying at these particular times, even though he prays with a good intention, since he prays to Allah. However, when the prayer at a particular time coincides with the time at which the polytheists prostrate to the sun, we are forbidden from praying at that specific time to avoid resembling them.

There is no doubt that standing up in honor of a national anthem or flag is a habit that infiltrated into Muslim society. This habit came to us from the Western or Eastern non-Muslims, and it is not the way of the believers. So it is not permissible for a Muslim to do it, even with a good intention.

The fatwa issued by the Standing Committee for Scientific Research and Issuing Fataawa reads:

"It is not permissible for a Muslim to stand in honor of a national anthem or flag; this is a repudiated Bid‘ah (innovation in religion) as it did not happen during the time of the Prophet or that of the Rightly-Guided Caliphs, may Allah be pleased with them. It also runs counter to the perfection of due, pure Tawheed (monotheism) and to the sincerity of dedicating all glorification to Allah Alone. It is also a means that can lead to Shirk (associating partners with Allah in His Divinity or worship) and involves imitation of the disbelievers and their bad customs as well as their excessive glorification of their leaders and their exaggerated ceremonies. The Prophet prohibited resembling or imitating the disbelievers."

For more benefit, please refer to fatwas 17689, 83792 and 85728.

As regards the wearing of clothes that bear the names or pictures of the players who are mentioned in the question or other disbelievers, then there are several violations of Shariah in wearing them, including the promotion of those infidels, praising them, and loving them, and this is contrary to what we are ordered by our Shariah: to hate them and declare our disassociation from them. Allah says (what means): {You will not find a people who believe in Allah and the Last Day having affection for those who oppose Allah and His Messenger, even if they were their fathers or their sons or their brothers or their kindred...} [Quran 58:22]

The Tafseer (exegesis) of Adhwaa’ Al-Bayaan reads, "{having affection for those who oppose Allah and His Messenger} means loving and being allies to the enemies of Allah and His Messenger…"

Besides, Ash-Shawkaani said in Fat-h Al-Qadeer, "i.e.: loving and being loyal to those who are opposed to and are enemies of Allah and His Messenger … Faith deters this and forbids it, and protecting one’s faith is more important than protecting one’s fathers or sons or brothers or kindred."

Hating the disbelievers for the sake of Allah is very unlikely to be accompanied by wearing clothes that bear their pictures or names by one who knows the objective for which their pictures are drawn and their names are inscribed and what it entails of promoting and publicizing them, while the person is not obliged to wear them.

Also, among the violations of Shariah in wearing clothes that have the pictures of those players is that wearing them is a form of deceiving and misleading the Muslim youth and making them love them in their hearts and urging them to imitate them.

Allah knows best.