Whether you realize it or not, dividend stocks are the under-the-radar workhorse of many successful investment portfolios. According to a report from JP Morgan Asset Management that was published in May 2013, dividend stocks that initiated and grew their payout returned an average of 9.5% per year between 1972 and 2012, which compared to an average annual return of a meager 1.6% over the same period for non-dividend-paying stocks. 

Aside from helping investors outperform, a regular dividend payout can also help to calm the nerves of skittish investors, and can be reinvested, via a dividend reinvestment plan (Drip), back into more shares of dividend-paying stock to rapidly compound wealth. Drips are what practically all top-tier money managers use the increase the wealth of their clients over the long term.

But it's important to note that great dividend stocks aren't only found in the United States. Looking beyond our borders, there are three high-yield international dividend stocks that could deliver impressive returns and sustainable income for the foreseeable future.

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Mobile TeleSystems: 9.6% yield

The highest-yielding company on this list is Russian telecom giant Mobile TeleSystems (NYSE:MBT), which is also known as MTS.

Publicly traded telecom companies are certainly no stranger to paying out a healthy dividend. Operating a subscription-based model dependent on wireless contracts, telecom providers tend to have predictable cash flow and operating expenses, thereby allowing them to pay out a notable portion of their earnings as a dividend to shareholders.

What makes Mobile TeleSystems so intriguing is that investors mostly seem to be ignoring the company's potential with regard to the rollout of 5G and 4G LTE networks. You see, wireless saturation in Russia is very high, and there's obvious concern as to how MTS will grow its top and bottom line with wireless penetration already well above 100% throughout the country. The answer relates to the upcoming smartphone upgrade cycle. As MTS prepares to introduce 5G networks in major cities, as well as upgrades burgeoning cities throughout the vast country, it provides an opportunity for the company to reap the rewards of higher data usage. And make no mistake, data is a high-margin growth driver for Mobile TeleSystems.

Additionally, MTS has been branching off into other industries. Having incorporated MTS Bank, the company now boasts fintech solutions and cloud services. On a year-over-year basis, the company's retail loan portfolio for MTS Bank grew by 85% in the third quarter, with the bank now on the cusp of its 2019 target of having 2 million end-of-year customers. 

In short, Mobile TeleSystems operates a business that can deliver mid-single-digit growth, is valued at less than nine times next year's earnings per share, and is paying out a nearly 10% yield. That's something investors can definitely subscribe to.

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Bank of Nova Scotia: 4.8% yield

The banking industry is known for being a moneymaker. This is why Warren Buffett has more than 45% of his portfolio tied up in the financial sector, most of which is devoted to money center banks. But you might be surprised to learn that Canada has some of the steadiest-growing and financially sound banks in the world, which is why Bank of Nova Scotia (NYSE:BNS) (known better as Scotiabank) should be on income investors' lists.

To begin with, Scotiabank has predominantly focused on the bread and butter of banking, rather than being tempted by riskier means of generating income. In the fiscal fourth quarter of 2019, Scotiabank saw loan and deposit growth across the board. In Canada, deposits improved 4% with loan growth of 5%, while internationally Bank of Nova Scotia delivered 10% deposit growth and 8% loan growth. Although a lower-yielding environment tends to work against banks as deposits increase, Scotiabank still managed 5% net interest income growth in Canada in full-year 2019, with noninterest income (bank fees and such) leading the way internationally.

That brings me to my next point: Bank of Nova Scotia has plenty of opportunity to grow its business in overseas markets. In spite of some higher expensing tied to its expansion, including technology upgrades and acquisitions, Scotiabank ended the year with market share of 7.6% in Mexico, 18.2% in Peru, 14.3% in Chile, and 6% in Colombia. Most impressively, its return on equity for full-year 2019 was 20% in Mexico and 26% in Peru, which demonstrates the pull these potentially faster-growing Latin and South American economies can have on its income statement. 

Also, similar to MTS, Bank of Nova Scotia is relatively inexpensive. At 9.6 times Wall Street's fiscal 2020 EPS estimates, Scotiabank is valued at roughly 10% below its five-year historic average for forward earnings. Suffice it to say that this 4.8% yield is something income investors can bank on for years to come.

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GlaxoSmithKline: 4.3% yield

Another high-yield international dividend stock that could be the cure to your income ills is U.K.-based GlaxoSmithKline (NYSE:GSK) and its 4.3% yield.

Keeping with the theme of abbreviations on this list, GSK, as Glaxo is best known, is attractive because each of its three operating segments brings something to the table that the other segments lack. As an example, consumer healthcare is traditional a slower-growing segment, but it offers strong pricing power and highly predictable cash flow. Vaccines, while faster-growing of late, tend to provide steady growth potential as the global population expands. Then there's pharmaceuticals, which have a finite period of exclusivity, but offer the juiciest margins.

Within pharmaceuticals, GSK has two highly profitable franchises. First, there's the company's next-generation respiratory therapies for COPD and asthma. Respiratory meds delivered 19% constant currency growth during the third quarter from the prior-year period with Trelegy Ellipta and Anoro Ellipta providing the bulk of the gains. The other significant growth driver in recent years has been HIV therapies, with Tivicay and Triumeq bringing in $1.44 billion combined in just the most recent quarter. 

While there's certain to be some pressure placed on GlaxoSmithKline's HIV therapies in a highly competitive indication, it's important to note that these core indication of COPD, asthma, and HIV aren't curable. That provides a long runway for GSK to improve the lives of its patients, as well as improve the financial outlook of its shareholders.