Filipino or Tagalog? Is there really a difference? Sometimes, we just shrug this question off. It's easy to say these two don't have a difference since native speakers of these languages will not be strangers when they talk. They will understand each other completely.
But technical translations demand more literal transfers. Consequently whether to use Filipino or Tagalog becomes a conscious question.
The following are not rules but options:
When source words are without exact equivalents in Tagalog, here is where Filipino becomes useful. In a sense, one often resorts to using Filipino when "pure Tagalog" expressions can't be found. A translation therefore cannot be purely Filipino or Tagalog, because there is yet no clear line that distinguishes one from the other.
Tagalog is not a dialect but a major language in the Philippines. Within the Tagalog region, there are many dialects such as the variations found in Nueva Ecija, Bulacan, Mindoro, Palawan, Quezon, Rizal and Batangas provinces. Ninety percent of native Tagalog speakers are born and bred and grew up in these provinces.
Filipino is based on Tagalog. Without Tagalog, I doubt if there will ever be a clear identification of the Filipino language. On second thought, maybe, Filipino will be based on Cebuano, or Ilocano, or Hiligaynon, or Bicolano which are also major languages. Some Cebuanos are sometimes jealous because majority of the so-called Filipino words and expressions are actually Tagalog. But in Davao, Cebuano is mixed liberally with Tagalog, and this probably accounts for the difference between Cebuano and the so-called Cebuano-Davao.
Filipino or Tagalog? Some Suggested Differences
for the Translation of Technical Documents
"Filipino" incorporates more words and borrowings from other major Philippine languages including Visayan, Ilocano, Bicolano, Ilongo, Waray, Kapampangan, Pangasinense, Maranao, and also from languages outside the Philippines. But remember that "Tagalog" has borrowed words and expressions from Spanish, English, Chinese, Malay and others. As an accepted practice, what has already been borrowed and in use widely in the Tagalog region for a long time can be safely categorized as Tagalog, but new borrowings and word-mixes can be identified as Filipino. The origin of these Filipino or Tagalog words can't always be determined, but academicians, writers, and other word-crafters, invent new words in order to accommodate the fast pace of world events, pop culture and new media. Most expressions, need to be "Filipinized" using Filipino rules of spelling.
Filipino and Manila Tagalog
Most Tagalog native speakers (even with their dialect variations) can translate into Filipino if they know these differences, but I strongly believe that there is a lesser number of Filipino language speakers (Tagalogs, Ilocanos, Bicolanos, Cebuanos, Warays, Kapampangans etc.) who can translate fluently into "strictly" pure Tagalog. (Some will ask, "Is there such a thing as pure Tagalog?")
Filipino is heard mostly in Manila where Filipinos of different Philippine ethnic origins and languages merge. In Manila, foreigners who are armed with their knowledge of pure (?) Tagalog will eventually lose their acquired fluency in this language since more people will prefer to speak to them in English or Taglish anyway (even if they are Germans, or Italians, or Japanese or Koreans).
Or when a tourist travels around the non-Tagalog provinces and speaks in her learned Tagalog, those she talks to will probably respond to her in a mixture of their local language and some Filipino. Everybody in the Philippines who goes to public or private school is mandated to learn Filipino; that is, whatever his or her ethnic origin. This mandate naturally yields combinations of inter-regional expressions brought about by language adjustments. The resulting Filipino or Tagalog expressions further enrich the language and make it even more useful around the country.
Whether to translate into Filipino or Tagalog becomes an issue only when there's a strict demand to use one or the other.