Keyword research is one of the core fundamentals of search marketing and of international SEO. The same principles apply to international keyword research as to when it is carried out for a single language such as US English.
However, the following new challenges arise:
The solution is that you must use a mother-tongue speaker of the language to carry out the research (not someone who studied the language at school or night class) and who is ALSO trained in the requirements of search marketing to present the right findings.
Ask the researcher to also give you a 'back translation' of the keywords, so that both you and the people involved in the evaluation process can understand what's going on.
You need to specify how you wish misspellings and grammatical issues to be treated. You'll need to look at how your brands are handled. In many languages it's common for English terms to creep into searches - be aware that these may actually mean something different to how you understand those English terms. Additionally, in languages such as Arabic and Russian you may find that your brands are transliterated into the local character set. In other words, the local character set is used to represent brands phonetically - you'll need to be advised about these.
Bear in in mind that a keyword in English may turn in no direct equivalents in some languages or multiple similar terms in another.
Long tail applies in all language to some extent - but it does vary from one language to another. Don't expect the number of keywords to be the same in all languages - that won't be the case.
To help the keyword researcher, provide a list of successful SEED terms in your start language. These are to be used for the inspiration of the researcher, NOT to be translated. Translation of keywords will almost always result in terms with low search volumes and they may also look or sound ridiculous in the target language.
For some languages, you may need to use multiple spellings of the same keyword because of the way that language works eg Finnish or Russian. You may also need to vary the content included locally to provide a suitable platform for your selected keywords.